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, 43 insertions, 0 deletions

diff --git a/tests/meson.build b/tests/meson.build
index 4c02ccd..76d4962 100644
--- a/tests/meson.build
+++ b/tests/meson.build
@@ -9,6 +9,17 @@
 # Coverage:        meson setup -Db_coverage=true build
 #                  meson test -C build
 #                  ninja -C build coverage   (needs lcov + genhtml)
+#
+# Fixture dirs are passed to tests via env vars:
+#   GGAZE_FIXTURES_DIR  committed tests/fixtures/ (CI-portable baseline)
+#   GGAZE_SAMPLE_DIR    local ./sample-images (not git-tracked; tests skip
+#                       cleanly when absent so CI stays green)
+
+fixtures_env = environment()
+fixtures_env.set('GGAZE_FIXTURES_DIR',
+   join_paths(meson.project_source_root(), 'tests', 'fixtures'))
+fixtures_env.set('GGAZE_SAMPLE_DIR',
+   join_paths(meson.project_source_root(), 'sample-images'))
 
 # --- unit track ---
 test_bootstrap = executable(
@@ -34,6 +45,26 @@ test('app', test_app,
    depends : ggaze_exe,
 )
 
+test_detect = executable(
+   'test_detect',
+   ['test_detect.c', ggaze_conf_h],
+   include_directories : [inc, src_inc],
+   dependencies : [glib_dep],
+   link_with : ggaze_lib,
+   install : false,
+)
+test('detect', test_detect, suite : 'unit', env : fixtures_env)
+
+test_loader_pixbuf = executable(
+   'test_loader_pixbuf',
+   ['test_loader_pixbuf.c', ggaze_conf_h],
+   include_directories : [inc, src_inc],
+   dependencies : [gdkpixbuf_dep, gtk4_dep, glib_dep, gio_dep],
+   link_with : ggaze_lib,
+   install : false,
+)
+test('loader_pixbuf', test_loader_pixbuf, suite : 'unit', env : fixtures_env)
+
 # --- integration track (needs a display; CI uses xvfb-run) ---
 test_window = executable(
    'test_window',
@@ -45,4 +76,16 @@ test_window = executable(
 )
 test('window', test_window, suite : 'integration')
 
+test_open_and_show = executable(
+   'test_open_and_show',
+   ['test_open_and_show.c', ggaze_conf_h],
+   include_directories : [inc, src_inc],
+   dependencies : ggaze_deps,
+   link_with : ggaze_lib,
+   install : false,
+)
+test('open_and_show', test_open_and_show,
+   suite : 'integration',
+   env : fixtures_env)
+
 subdir('integration')
\ No newline at end of file