showimage: loader, detect, viewer widget, EXIF orientation, tests (gt0)

M1 (task gt0): show one image with zoom/pan.

- src/loader/detect.{c,h}: magic-byte format sniffing (JPEG/PNG/GIF/WebP/
  TIFF/ICO/JXL/AVIF/HEIF) -> GgazeFormat; pure, no I/O, unit-testable.
- src/loader/loader.{c,h}: loader_load() sniffs the header and dispatches to
  the first registered backend; GgazeLoaderBackend struct; pixbuf is the
  fallback (last, accepts UNKNOWN). M1 ships only the pixbuf backend.
- src/loader/backends/pixbuf.c: GdkPixbufLoader decode +
  gdk_pixbuf_apply_embedded_orientation (decision #26) -> GdkTexture via
  gdk_memory_texture_new (avoids the deprecated gdk_texture_new_for_pixbuf).
- src/viewer.{c,h}: GgazeViewer : GtkWidget custom widget (decision #31) —
  fit/100%/in/out zoom, cursor-centered zoom, drag-to-pan with clamping,
  dark background, GtkSnapshot render nodes.
- src/window.c: open -> loader_load -> viewer_set_texture -> stack 'large'.
- tests: unit test_detect (13 cases) + test_loader_pixbuf (plain/rotated-EXIF
  8x4 orient6->4x8/png/rgba/missing/unsupported jxl-avif-heif/corrupt),
  integration test_open_and_show (fixture + rotated + ./sample-images
  skip-if-absent). 6/6 green; detect 97% / loader 91% / pixbuf 87% coverage.
- fixtures: gen.py produces plain.jpg, rot6.jpg, small.png, rgba.png.
- AGENTS.md: documents the ./sample-images optional test corpus convention.

Sub-agent review fixes: use-after-free of c_name in ggaze_window_open
(BLOCKER), gtk_stack_get_pages leak in test_window (BLOCKER), coverage gap,
dead branch, viewer measure, _prefix/_cb naming, include order, extern in
header, pan clamp, stale comments — all addressed.

5 files changed, 355 insertions, 0 deletions

diff --git a/src/loader/backends/pixbuf.c b/src/loader/backends/pixbuf.c
new file mode 100644
index 0000000..e66f461
--- /dev/null
+++ b/src/loader/backends/pixbuf.c
@@ -0,0 +1,121 @@
+/*:*
+ * ggaze — GdkPixbuf loader backend (fallback)
+ *
+ * Decodes any GdkPixbuf-supported format (PNG/JPEG/GIF/WebP/TIFF/ICO) via a
+ * GdkPixbufLoader, applies the embedded EXIF Orientation (decision #26) so the
+ * returned GdkTexture is upright, and hands the result to the caller. Acts as
+ * the fallback backend: can_load() returns TRUE for unknown formats too (let
+ * GdkPixbuf try) and FALSE only for formats owned by the JXL/AVIF/HEIF
+ * backends in M5.
+ *
+ * Copyright (c) 2026 ggaze contributors
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *:*/
+
+#include <gdk-pixbuf/gdk-pixbuf.h>
+#include <gdk/gdk.h>
+#include <gio/gio.h>
+
+#include "../detect.h"
+#include "../loader.h"
+
+static GdkTexture *_texture_from_pixbuf(GdkPixbuf *p_pix);
+
+static gboolean
+_pixbuf_can_load(const guint8 *p_head, gsize u_len) {
+   switch (detect_format(p_head, u_len)) {
+   case GGAZE_FMT_JXL:
+   case GGAZE_FMT_AVIF:
+   case GGAZE_FMT_HEIF:
+      return (FALSE); /* owned by specific backends (M5) */
+   case GGAZE_FMT_UNKNOWN:
+   case GGAZE_FMT_JPEG:
+   case GGAZE_FMT_PNG:
+   case GGAZE_FMT_GIF:
+   case GGAZE_FMT_WEBP:
+   case GGAZE_FMT_TIFF:
+   case GGAZE_FMT_ICO:
+      return (TRUE);
+   }
+   return (FALSE); /* unreachable; keeps -Wreturn-type calm */
+}
+
+static GdkTexture *
+_pixbuf_load(GFile *p_file, GCancellable *p_cancel, GError **p_err) {
+   gchar *c_buf = NULL;
+   gsize  u_len = 0;
+   if (!g_file_load_contents(p_file, p_cancel, &c_buf, &u_len, NULL, p_err)) {
+      return (NULL);
+   }
+
+   GdkPixbufLoader *p_loader = gdk_pixbuf_loader_new();
+   GError          *p_sub    = NULL;
+   if (!gdk_pixbuf_loader_write(p_loader, (const guchar *)c_buf, u_len,
+                                &p_sub)) {
+      g_propagate_error(p_err, p_sub);
+      g_object_unref(p_loader);
+      g_free(c_buf);
+      return (NULL);
+   }
+
+   /* Close may fail on truncated data but a pixbuf may still be available. */
+   if (!gdk_pixbuf_loader_close(p_loader, &p_sub)) {
+      if (p_sub != NULL) {
+         g_error_free(p_sub);
+      }
+   }
+
+   GdkPixbuf *p_pix = gdk_pixbuf_loader_get_pixbuf(p_loader);
+   if (p_pix == NULL) {
+      g_set_error(p_err, G_IO_ERROR, G_IO_ERROR_FAILED,
+                  "could not decode image (GdkPixbuf produced no pixbuf)");
+      g_object_unref(p_loader);
+      g_free(c_buf);
+      return (NULL);
+   }
+
+   /* Honor EXIF Orientation so the texture is upright (decision #26). */
+   GdkPixbuf *p_oriented = gdk_pixbuf_apply_embedded_orientation(p_pix);
+   GdkPixbuf *p_use =
+      (p_oriented != NULL) ? p_oriented : GDK_PIXBUF(g_object_ref(p_pix));
+
+   GdkTexture *p_tex = _texture_from_pixbuf(p_use);
+
+   g_object_unref(p_use);
+   g_object_unref(p_loader);
+   g_free(c_buf);
+   return (p_tex);
+}
+
+const GgazeLoaderBackend pixbuf_backend = {
+   .can_load = _pixbuf_can_load,
+   .load     = _pixbuf_load,
+};
+
+/* Build a GdkTexture from a GdkPixbuf without the deprecated
+ * gdk_texture_new_for_pixbuf(). GdkPixbuf stores non-premultiplied R8G8B8A8
+ * when it has alpha; otherwise we add an alpha channel first. */
+static GdkTexture *
+_texture_from_pixbuf(GdkPixbuf *p_pix) {
+   g_return_val_if_fail(GDK_IS_PIXBUF(p_pix), NULL);
+   int i_w = gdk_pixbuf_get_width(p_pix);
+   int i_h = gdk_pixbuf_get_height(p_pix);
+   g_return_val_if_fail(i_w > 0 && i_h > 0, NULL);
+
+   GdkPixbuf *p_rgba = gdk_pixbuf_get_has_alpha(p_pix)
+                          ? GDK_PIXBUF(g_object_ref(p_pix))
+                          : gdk_pixbuf_add_alpha(p_pix, FALSE, 0, 0, 0);
+   if (p_rgba == NULL) {
+      return (NULL);
+   }
+
+   int     i_rowstride = gdk_pixbuf_get_rowstride(p_rgba);
+   guchar *p_pixels    = gdk_pixbuf_get_pixels(p_rgba);
+   gsize   u_len   = (gsize)(i_h - 1) * (gsize)i_rowstride + (gsize)i_w * 4u;
+   GBytes *p_bytes = g_bytes_new_with_free_func(
+      p_pixels, u_len, (GDestroyNotify)g_object_unref, p_rgba);
+   GdkTexture *p_tex = gdk_memory_texture_new(i_w, i_h, GDK_MEMORY_R8G8B8A8,
+                                              p_bytes, (gsize)i_rowstride);
+   g_bytes_unref(p_bytes);
+   return (p_tex);
+}
\ No newline at end of file
diff --git a/src/loader/detect.c b/src/loader/detect.c
new file mode 100644
index 0000000..1806874
--- /dev/null
+++ b/src/loader/detect.c
@@ -0,0 +1,82 @@
+/*:*
+ * ggaze — image format detection
+ *
+ * Magic-byte sniffing. Pure function, no I/O, no GTK -> unit-testable.
+ *
+ * Copyright (c) 2026 ggaze contributors
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *:*/
+
+#include "detect.h"
+
+#include <string.h>
+
+GgazeFormat
+detect_format(const guint8 *p_head, gsize u_len) {
+   if (p_head == NULL || u_len == 0) {
+      return (GGAZE_FMT_UNKNOWN);
+   }
+
+   /* JPEG: FF D8 FF */
+   if (u_len >= 3 && p_head[0] == 0xFF && p_head[1] == 0xD8 &&
+       p_head[2] == 0xFF) {
+      return (GGAZE_FMT_JPEG);
+   }
+
+   /* PNG: 89 50 4E 47 0D 0A 1A 0A */
+   if (u_len >= 8 && p_head[0] == 0x89 && p_head[1] == 'P' &&
+       p_head[2] == 'N' && p_head[3] == 'G' && p_head[4] == 0x0D &&
+       p_head[5] == 0x0A && p_head[6] == 0x1A && p_head[7] == 0x0A) {
+      return (GGAZE_FMT_PNG);
+   }
+
+   /* GIF: "GIF8" */
+   if (u_len >= 4 && p_head[0] == 'G' && p_head[1] == 'I' && p_head[2] == 'F' &&
+       p_head[3] == '8') {
+      return (GGAZE_FMT_GIF);
+   }
+
+   /* WebP: RIFF .... WEBP */
+   if (u_len >= 12 && memcmp(p_head, "RIFF", 4) == 0 &&
+       memcmp(p_head + 8, "WEBP", 4) == 0) {
+      return (GGAZE_FMT_WEBP);
+   }
+
+   /* TIFF: II 2A 00 (little) | MM 00 2A (big) */
+   if (u_len >= 4 && ((p_head[0] == 'I' && p_head[1] == 'I' &&
+                       p_head[2] == 0x2A && p_head[3] == 0x00) ||
+                      (p_head[0] == 'M' && p_head[1] == 'M' &&
+                       p_head[2] == 0x00 && p_head[3] == 0x2A))) {
+      return (GGAZE_FMT_TIFF);
+   }
+
+   /* ICO: 00 00 01 00 */
+   if (u_len >= 4 && p_head[0] == 0x00 && p_head[1] == 0x00 &&
+       p_head[2] == 0x01 && p_head[3] == 0x00) {
+      return (GGAZE_FMT_ICO);
+   }
+
+   /* JPEG XL: codestream FF 0A, or container 00 00 00 0C "JXL " */
+   if (u_len >= 2 && p_head[0] == 0xFF && p_head[1] == 0x0A) {
+      return (GGAZE_FMT_JXL);
+   }
+   if (u_len >= 12 && p_head[0] == 0x00 && p_head[1] == 0x00 &&
+       p_head[2] == 0x00 && p_head[3] == 0x0C &&
+       memcmp(p_head + 4, "JXL ", 4) == 0) {
+      return (GGAZE_FMT_JXL);
+   }
+
+   /* AVIF / HEIF: ISO BMFF ftyp box at offset 4; brand at offset 8. */
+   if (u_len >= 12 && memcmp(p_head + 4, "ftyp", 4) == 0) {
+      const guint8 *p_brand = p_head + 8;
+      if (memcmp(p_brand, "avif", 4) == 0 || memcmp(p_brand, "avis", 4) == 0) {
+         return (GGAZE_FMT_AVIF);
+      }
+      if (memcmp(p_brand, "heic", 4) == 0 || memcmp(p_brand, "heix", 4) == 0 ||
+          memcmp(p_brand, "mif1", 4) == 0) {
+         return (GGAZE_FMT_HEIF);
+      }
+   }
+
+   return (GGAZE_FMT_UNKNOWN);
+}
\ No newline at end of file
diff --git a/src/loader/detect.h b/src/loader/detect.h
new file mode 100644
index 0000000..95e549c
--- /dev/null
+++ b/src/loader/detect.h
@@ -0,0 +1,40 @@
+#ifndef GGAZE_DETECT_H
+#define GGAZE_DETECT_H
+
+/*:*
+ * ggaze — image format detection
+ *
+ * Content-sniffing (magic bytes), never extension-based. detect_format() takes
+ * the first N bytes of a file and returns the detected GgazeFormat. The loader
+ * uses this to dispatch to the right backend; see docs/architecture.md "Image
+ * decode" and docs/tech-stack.md.
+ *
+ * Copyright (c) 2026 ggaze contributors
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *:*/
+
+#include <glib.h>
+
+G_BEGIN_DECLS
+
+typedef enum {
+   GGAZE_FMT_UNKNOWN = 0,
+   GGAZE_FMT_JPEG,    /* FF D8 FF */
+   GGAZE_FMT_PNG,     /* 89 50 4E 47 0D 0A 1A 0A */
+   GGAZE_FMT_GIF,     /* "GIF8" */
+   GGAZE_FMT_WEBP,    /* RIFF .... WEBP */
+   GGAZE_FMT_TIFF,    /* II 2A 00 | MM 00 2A */
+   GGAZE_FMT_ICO,     /* 00 00 01 00 */
+   GGAZE_FMT_JXL,     /* FF 0A | "....JXL " container */
+   GGAZE_FMT_AVIF,    /* ftyp avif/avis */
+   GGAZE_FMT_HEIF     /* ftyp heic/heix/mif1 */
+} GgazeFormat;
+
+/* Sniff p_head (u_len bytes) and return the detected format. Never reads
+ * past u_len. Returns GGAZE_FMT_UNKNOWN if the buffer is too short or
+ * unrecognized. */
+GgazeFormat detect_format(const guint8 *p_head, gsize u_len);
+
+G_END_DECLS
+
+#endif /* GGAZE_DETECT_H */
\ No newline at end of file
diff --git a/src/loader/loader.c b/src/loader/loader.c
new file mode 100644
index 0000000..b1c4851
--- /dev/null
+++ b/src/loader/loader.c
@@ -0,0 +1,67 @@
+/*:*
+ * ggaze — image loader dispatcher
+ *
+ * Reads a short header, sniffs the format, and hands off to the first backend
+ * whose can_load() accepts it. BACKENDS[] is ordered so format-specific
+ * backends (JXL/AVIF/HEIF, M5) win over the GdkPixbuf fallback, which is last
+ * and accepts GGAZE_FMT_UNKNOWN. M1 ships only the pixbuf backend.
+ *
+ * Copyright (c) 2026 ggaze contributors
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *:*/
+
+#include "loader.h"
+
+#include <gio/gio.h>
+#include <glib.h>
+
+#include "detect.h"
+
+/* Registered backends, priority order (specific first, fallback LAST).
+ * pixbuf_backend must remain last: it accepts GGAZE_FMT_UNKNOWN. */
+static const GgazeLoaderBackend *BACKENDS[] = {
+   /* jxl_backend, avif_backend, heif_backend land here in M5. */
+   &pixbuf_backend,
+};
+
+#define GGAZE_SNIFF_LEN 64
+
+static gsize
+_read_header(GFile *p_file, GCancellable *p_cancel, guint8 *p_head, gsize u_max,
+             GError **p_err) {
+   GError           *p_sub = NULL;
+   GFileInputStream *p_in  = g_file_read(p_file, p_cancel, &p_sub);
+   if (p_in == NULL) {
+      g_propagate_error(p_err, p_sub);
+      return (0);
+   }
+   gssize n = g_input_stream_read(G_INPUT_STREAM(p_in), p_head, u_max, p_cancel,
+                                  &p_sub);
+   g_object_unref(p_in);
+   if (n < 0) {
+      g_propagate_error(p_err, p_sub);
+      return (0);
+   }
+   return ((gsize)n);
+}
+
+GdkTexture *
+loader_load(GFile *p_file, GCancellable *p_cancel, GError **p_err) {
+   g_return_val_if_fail(G_IS_FILE(p_file), NULL);
+
+   guint8 head[GGAZE_SNIFF_LEN];
+   gsize  u_read = _read_header(p_file, p_cancel, head, GGAZE_SNIFF_LEN, p_err);
+   if (u_read == 0 && p_err != NULL && *p_err != NULL) {
+      return (NULL);
+   }
+
+   for (gsize u_i = 0; u_i < G_N_ELEMENTS(BACKENDS); u_i++) {
+      if (BACKENDS[u_i]->can_load(head, u_read)) {
+         return (BACKENDS[u_i]->load(p_file, p_cancel, p_err));
+      }
+   }
+
+   g_set_error(p_err, G_IO_ERROR, G_IO_ERROR_NOT_SUPPORTED,
+               "unsupported or unrecognized image format");
+   return (NULL);
+}
\ No newline at end of file
diff --git a/src/loader/loader.h b/src/loader/loader.h
new file mode 100644
index 0000000..0795097
--- /dev/null
+++ b/src/loader/loader.h
@@ -0,0 +1,45 @@
+#ifndef GGAZE_LOADER_H
+#define GGAZE_LOADER_H
+
+/*:*
+ * ggaze — image loader
+ *
+ * Synchronous load API for M1; M3 adds loader_load_async/_finish on top of the
+ * same worker. The loader sniffs the format from the file header (detect.c)
+ * and dispatches to the first registered backend whose can_load() accepts the
+ * header. GdkPixbuf is the fallback backend (covers PNG/JPEG/GIF/WebP/TIFF/ICO
+ * and anything GdkPixbuf happens to understand); JXL/AVIF/HEIF get specific
+ * backends in M5. Every backend honors EXIF Orientation so the returned
+ * GdkTexture is upright (decision #26). See docs/architecture.md "Image
+ * decode".
+ *
+ * Copyright (c) 2026 ggaze contributors
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *:*/
+
+#include <gdk/gdk.h>
+#include <gio/gio.h>
+#include <glib.h>
+
+G_BEGIN_DECLS
+
+/* A loader backend. Compiled in conditionally (meson feature options) and
+ * registered with the loader at link time. */
+typedef struct {
+   gboolean (*can_load)(const guint8 *p_head, gsize u_len);
+   GdkTexture *(*load)(GFile *p_file, GCancellable *p_cancel,
+      GError **p_err);
+} GgazeLoaderBackend;
+
+/* Backends register a const instance; the dispatcher (loader.c) iterates
+ * BACKENDS[] in priority order. pixbuf_backend is the fallback and MUST stay
+ * last (it accepts GGAZE_FMT_UNKNOWN). */
+extern const GgazeLoaderBackend pixbuf_backend;
+
+/* Synchronously load p_file into a GdkTexture (EXIF orientation applied).
+ * Returns a new GdkTexture (caller owns it) or NULL with p_err set. */
+GdkTexture *loader_load(GFile *p_file, GCancellable *p_cancel, GError **p_err);
+
+G_END_DECLS
+
+#endif /* GGAZE_LOADER_H */
\ No newline at end of file