walkdir: navigator, shortcuts, window wiring, drop target, tests (ht0)

M2 (task ht0): walk the directory.

- src/navigator.{c,h}: GObject (no GtkWidget) holding the folder listing —
  image-MIME filter + extension fallback, hide-RAW-sidecars (decision #33),
  sort name/time/size, cursor + prev/next/wrap, path-based marks (survive
  re-sort, pruned on rescan/monitor, cleared on remove), nearest-fallback,
  GFileMonitor debounced 250ms (decision #28), 'changed' signal. 92% gcov.
- src/shortcuts.{c,h}: GtkShortcutController (MANAGED) binding h/l/Left/Right/
  g/G/o/q to win.* actions; one table all milestones add to.
- src/window.{c,h}: open file->parent+current / folder->first; h/l/g/G actions
  -> nav -> load -> viewer; single GCancellable last-write-wins (g_file_equal
  guard for M3 async); header title 'filename · n/total'; GtkDropTarget
  (GDK_TYPE_FILE_LIST, many->first, decision Z); GtkFileDialog open.
- data/gschema: add hide-raw-sidecars key.
- tests: unit test_navigator (12 subtests), integration test_walk_folder
  (temp fixtures walk + ./sample-images skip-if-absent). 8/8 green; ASan clean
  (GIO_USE_VFS=local in fixtures_env).

Fixed during this milestone: a UAF (borrowed c_name used after unref), a
cancellable double-ref leak, time::modified UINT64 vs INT64, and several
test-harness leaks. Sub-agent review issues addressed (mark pruning on
rescan/monitor, remove-before-cursor test, recursive temp-dir cleanup,
callback _cb naming, connect-before-mutate).

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