Let a project keep named snapshots and restore them (#231)

Autosave only keeps the latest revision. Before a risky change there
was no way to put a known good state aside and go back to it later.
The main menu now has "Snapshots..." to create, restore and delete
named copies of the open project.

- Server: SnapshotStore.php keeps each snapshot as a copy of the
  project JSON under data/snapshots/<project>/ (git-ignored). Four
  routes under projects/{id}/snapshots use the same owner check as the
  project, so an admin also sees foreign snapshots.
- Create writes the autosave first. With unsaved edits (play with open
  edits, failed save, conflict) no snapshot is made and a message says
  why. The rule is the pure function in src/editor/snapshots.ts.
- An empty name falls back to the local date and time.
- Restore asks first, then keeps the current state as "Before
  restoring <name>". It writes the snapshot back as a new revision and
  reloads the project, so the undo list is empty. A restore can itself
  be undone by restoring that snapshot.
- Deleting a project deletes its snapshots too. Images, sounds, fonts
  and pixel art are content-addressed and never deleted, so a snapshot
  can always reach them.
- Client: SnapshotStore and HttpSnapshotStore in src/core/store, the
  SnapshotsDialog, and the flows in ProjectSession. A stale revision
  (409) becomes StaleRevisionError.
- CI runs the new PHP test in the gate job next to accounts.

Not covered by e2e: the lock during play and the conflict case. Both
are unit tests only.

Tests:
- check.sh: 164 files, 2401 tests, CHECK EXIT 0.
- tests/php/snapshots.test.php: 14 checks, all passed.
- tests/editor/snapshots.test.ts: 5 tests for the time format, the
  blocker rule, the routes and 409.
- New e2e module 50 on a throwaway project: create, rename, restore,
  undo the restore, delete, then delete the project and get 404 on
  its snapshots. 46b expects the new menu entry. e2e 46,50: 20 checks
  passed. The check sum goes from 1385 to 1393 (computed).
- Counter-checks, originals restored and compared with cmp: blocker
  always allowed fails 1 unit test. No sorting fails 2 PHP checks.
  Restore without keeping the old state fails 50c and aborts 50d.
- The first sort test with 2 snapshots passed by chance half the time
  because of random ids. It now uses 8.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitignore

@@ -3,6 +3,7 @@
 /data/e2e-password.txt
 /data/e2e-admin-password.txt
 /data/sessions/
+/data/snapshots/
 /data/users.json
 /data/textures/
 /data/fonts/