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.

1 files changed, 82 insertions, 0 deletions

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);
+}
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