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, 15 insertions, 7 deletions

diff --git a/meson.build b/meson.build
index e21b70c..b496e25 100644
--- a/meson.build
+++ b/meson.build
@@ -17,10 +17,11 @@ project('ggaze', 'c',
 )
 
 # --- Always-required dependencies (the core viewer needs all of these) ---
-gtk4_dep    = dependency('gtk4')
-glib_dep    = dependency('glib-2.0')
-gio_dep     = dependency('gio-2.0')
-adwaita_dep = dependency('libadwaita-1')
+gtk4_dep       = dependency('gtk4')
+glib_dep       = dependency('glib-2.0')
+gio_dep        = dependency('gio-2.0')
+adwaita_dep    = dependency('libadwaita-1')
+gdkpixbuf_dep  = dependency('gdk-pixbuf-2.0')  # pixbuf loader backend
 
 # --- Optional backends (feature: auto/disabled/enabled). See meson_options.txt.
 gegl_dep = dependency('gegl-0.4', required : get_option('gegl'))
@@ -52,13 +53,20 @@ src_inc = include_directories('src')
 #     duplicating sources). Plain-C modules will join this library as they
 #     land (navigator, trash, mover, ...).
 ggaze_lib = static_library('ggaze',
-   files('src/app.c', 'src/window.c'),
+   files(
+      'src/app.c',
+      'src/window.c',
+      'src/viewer.c',
+      'src/loader/loader.c',
+      'src/loader/detect.c',
+      'src/loader/backends/pixbuf.c',
+   ),
    include_directories : [inc, src_inc],
-   dependencies : [gtk4_dep, glib_dep, gio_dep, adwaita_dep],
+   dependencies : [gtk4_dep, glib_dep, gio_dep, adwaita_dep, gdkpixbuf_dep],
    install : false,
 )
 
-ggaze_deps = [gtk4_dep, glib_dep, gio_dep, adwaita_dep]
+ggaze_deps = [gtk4_dep, glib_dep, gio_dep, adwaita_dep, gdkpixbuf_dep]
 
 subdir('src')
 subdir('data')