enhance: wire up GEGL image filters (a cycle / s save) + fix export bugs

The enhancer module existed (GEGL presets: Auto-fix, Brightness, Contrast,
Saturation, Warm, Cool, Sharpen, Denoise) but was never connected to the
app and GEGL was never initialized. Wire it up:

- app.c: gegl_init() once in GApplication::startup (GEGL-gated).
- enhancer.c: add enhancer_load(GFile) -> GeglBuffer (gegl:load) and
  enhancer_buffer_to_texture(GeglBuffer) -> GdkTexture (RGBA8 ->
  GdkMemoryTexture) for the live preview bridge.
- enhancer_export: pick the saver from the output extension (jpg/png/webp)
  instead of always gegl:jpg-save (ju0 - was writing JPEG bytes into .png);
  verify the save actually produced a non-empty newer file instead of
  trusting g_file_test(EXISTS) on a pre-existing path (ku0 - false success).
- window.c: win.enhance (a) cycles the active preset and previews it on the
  current image (switches to large view, loads via GEGL, applies, shows the
  texture); win.enhance-save (s) exports <stem>-enhanced.<ext> with the
  active preset, never overwriting the original. Preview resets on
  navigation; the title shows the active preset.
- shortcuts.c: a -> win.enhance, s -> win.enhance-save; ? overlay Enhance
  group.
- tests: test_enhancer gains load_and_to_texture, export_format (PNG/JPEG
  signature checks), export_real_success (ku0 - parent-missing and
  pre-existing-directory both return FALSE). LSAN suppressions cover
  GEGL's jpg-load plugin leak. test_shortcut full-table updated (25 rows).

Synchronous apply (may briefly block on large images) and a navigate-away
dirty prompt are deliberately out of scope for v1.

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