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, 40 insertions, 0 deletions

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