Integrate navigator marks into window and grid UI (M8)

Wire the navigator mark API into the window actions, grid badges, and
keyboard/pointer controls:

- Navigator: add a path-based range anchor (p_last_mark) set on `v`
  toggle-on and cleared on unmark-of-anchor / clear_marks / remove /
  mark_removed / rescan-prune; expose navigator_get_last_mark().
  navigator_mark_removed now clears the removed file's mark (decision Q
  / task du0) and the anchor if it was it, without touching unrelated
  marks, and emits "changed" whenever the removed set or marks actually
  changed so grid badges and the header never go stale.
- Window: add win.mark-range (`V`: range from the last-mark anchor to the
  current/grid-selected image), register it, wire Shift+V, and document
  it in the shortcuts overlay. Trash/delete clear marks via mark_removed.
- Grid: middle-click on a cell selects it and dispatches win.mark
  (pointer-accessible marks).
- Tests: unit test_marks_anchor_and_remove and
  test_marks_anchor_pruned_on_rescan; integration test_marks.c covering
  toggle, all+clear, range, persistence across view switch,
  clear-on-trash, and the middle-click gesture wiring; updated
  test_shortcut.c action table + count.

clang-format clean; full suite + ASan leak check green.

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