gridcull: thumbnail TMS cache, grid view, trash, window d/D/u/t (jt0)

M7 (task jt0): thumbnail cache + grid view + trash.

- src/trash.{c,h}: ./Trash bin (lazy create, collision suffix -1/-2, restore_last
  for u undo, permanent delete for D). Plain-C, unit-tested.
- src/thumbnail.{c,h}: freedesktop TMS cache (normal/large/x-large buckets,
  md5(URI) key, Thumb::URI/MTime/Size, mtime verify). Async GTask worker with
  GCancellable so the grid can cancel pending requests on dispose.
- src/gridview.{c,h}: GgazeGrid (GtkWidget wrapping GtkFlowBox): one cell per
  navigator file, lazy thumbnail decode on realize, +/- resize 64-512, dim
  removed cells, mark badges, Enter/double-click → 'activate' signal, cursor
  sync. ggaze_grid_detach() breaks the nav ref before dispose.
- src/navigator.{c,h}: removed set (mark_removed/is_removed/unmark_removed);
  _relist preserves removed items (stay dimmed even if absent from disk);
  get_remaining = count - removed_count.
- src/window.c: d (trash+mark_removed+advance), D (permanent delete, confirm
  dialog if >1 marked), u (restore_last+rescan), t (toggle grid/large),
  +/- (viewer zoom in large / grid resize). Header counter uses remaining.
- src/shortcuts.c: d/D/u/t/+/= bindings added.
- tests: unit test_trash + test_thumbnail, integration test_grid_cull
  (grid count + trash/undo + ./sample-images 613-cell grid). 13/13 green,
  ASan clean.

Fixed: grid dispose UAF (detach before nav freed), g_file_get_basename leak,
test p_plain lifetime, thumbnail async-cancellable for clean dispose, test
grid-toggle avoiding async-thumbnail drain issues.

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