Footage: one watcher thread that wakes the interface only when a file changed, instead of a 2 s poll

Settings ▸ Import ▸ Reload Footage Changed on Disk used to spawn a stat thread every two seconds
from the UI thread, request a repaint 100 ms later to collect it, and request another repaint every
2 s in between, for every project with items, focused or not: the whole interface redrew on a timer
for footage that hadn't changed.

`footage_reload::Watcher` is one long-lived worker: the interface tells it what to watch (recomputed
when the project revision or the preference changes) and whether the window is in front; it stamps
the files every 2 s while the window is in front, every 10 s while another app has the front, at once
on focus regain and on a new set, and hands the stamps over — calling `ctx.request_repaint()` — only
when one differs from its last look. Nothing watched means the thread sleeps. `apply_stamps` and the
reload semantics are unchanged; `start_scan`/`poll_scan`/`Session::footage_scan` are gone. No new
dependencies: the stat loop is also what footage on a network share relies on, where FSEvents would
not report another machine's writes.

Tests: the watcher wakes the interface for the first stamps and for each change and never for an
unchanged file; behind another app it rests until the window is back; nothing watched is nothing
stamped and a new set is reported whole (and dropping it ends the thread); the host test
`the_watcher_notices_a_footage_file_changed_on_disk` saves a PSD from "another app" and sees it
reloaded once.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/preferences.md

@@ -120,9 +120,11 @@ Commands (CLI, MCP, control channel):
 - `import.reportMissingFrames`: gaps in an image sequence's numbering are reported on import
   (they show as colour-bar placeholders, as in After Effects; see [footage.md](footage.md))
 - `import.autoReloadFootage`: Automatically Reload Footage, `off`, `nonSequence` (the default, as
-  in After Effects) or `all` (image sequences too). Every two seconds,
-  and as soon as the window comes back to the front, the desktop app compares each footage file's
-  size and modification time with what they were, and reloads what another app saved: a
+  in After Effects) or `all` (image sequences too). Every two seconds while its window is in
+  front (every ten seconds while another app has the front), and as soon as the window comes back
+  to the front, the desktop app compares each footage file's size and modification time with what
+  they were, on a worker thread that only wakes the interface when something changed, and reloads
+  what another app saved: a
   Photoshop file edited in PhotoCraft or Photoshop shows its new pixels without File ▸ Reload
   Footage. It is not an undo step and doesn't modify the project; a layer item stays on its layer
   (found again by name), and a file that can't be read yet (still being written) is tried again once it changes again.