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, 8 insertions, 0 deletions

diff --git a/AGENTS.md b/AGENTS.md
index f56a48a..8b3ac2d 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -42,6 +42,14 @@ Shared helpers go in `tests/helpers/`; fixtures in `tests/fixtures/` (grow per
 milestone). Integration suites land with the milestone that first makes a flow
 possible — see `docs/IMPLEMENTATION.md` "Planned integration suites".
 
+**Optional realistic corpus:** `./sample-images/` (a local, NOT git-tracked
+613-image camera dump: 601 JPEG + 12 PNG, sizes up to 46MB, varied EXIF) is
+the realistic test corpus for integration tests and leak-check sessions. Any
+test or scripted session that uses it MUST skip cleanly when the directory is
+absent (so CI, which only has `tests/fixtures/`, stays green) and MUST NOT
+mutate the corpus — work on temp copies. The committed `tests/fixtures/` are
+the CI-portable baseline; `./sample-images/` is a local supplement.
+
 ## Conventions (enforced)
 
 - `docs/coding-conventions.md` summarizes; the `c-best-practices` skill wins.