Store: close second-round races in F-Droid loads, APK reuse and pruning

- The locked publication step also refuses a v1 index once v2 was accepted, so
  an overlapping v1 fallback can't replace a v2 cache at an equal timestamp.
- A cached APK is touched before hashing; if it vanishes, it's downloaded again.
- Only the app prunes (at start and after store downloads), since claims are
  in-process; the CLIs never prune.
- The CLI joins background refreshes on error exits too.
- The Windows lock loop retries only contention errors.
The concurrent-publication test now uses real flock contention.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/apk-repos.md

@@ -71,6 +71,8 @@ Authenticated reduced indexes and APKs live under
 `frame_host.cache_dir('apk-sources')`; indexes refresh after 24 hours. An
 expired index is still served (marked stale in the store) while it refreshes in
 the background; a failed refresh is retried after 10 minutes.
+Only the running Frame Control app prunes cached APKs (at start and after store
+downloads); the command-line tools never do.
 The existing catalogue's unverified index cache is never treated as authenticated.
 
 Rollback protection: each repository's newest accepted signed index timestamp