fix(peer): settle current-protocol local state cleanup

The follow-up backlog had drifted into three settled peer/runtime issues: the
legacy game-list fallback contradicted the one-wire-version policy, the Tauri
shell still re-derived local install state from disk after peer snapshots, and
`Availability::Downloading` existed even though active operations are already
reported through a separate operation table.

Remove the legacy `AnnounceGames` request and fallback service. Discovery now
ignores peers that do not advertise the current protocol and a peer id, and
library changes are sent through the current delta path only. This keeps the
runtime aligned with the documented current-build-only interoperability model.

Make peer `LocalGamesUpdated` snapshots authoritative for local fields in the
Tauri database. The GUI-side catalog still owns static metadata such as names,
sizes, and descriptions, but downloaded, installed, local version, and
availability now come from the peer runtime instead of a second whole-library
filesystem scan. Snapshot reconciliation also pins the missing-begin and
missing-finish lifecycle cases in tests.

Collapse availability back to the settled `Ready` and `LocalOnly` states.
Aggregation now counts only `Ready` peers as download sources, and the frontend
no longer carries a dead `Downloading` enum value.

The core peer also exposes the small non-GUI hooks needed by scripted callers:
startup options for state and mDNS, a local-ready event, direct connection, peer
snapshots, and an explicit post-download install policy. Those hooks reuse the
same current protocol path and do not add compatibility shims.

Test Plan:
- `git diff --check`
- `just fmt`
- `just clippy`
- `just test`

Refs: BACKLOG.md, FINDINGS.md, IMPL_DECISIONS.md

crates/lanspread-peer/ARCHITECTURE.md

@@ -165,7 +165,7 @@ Most scans become O(number of game dirs), with full recursion only when needed.
      immediate TCP/QUIC roundtrips when nothing changed.
    - Add a lightweight handshake in `run_peer_discovery` that exchanges
      `Hello`/`HelloAck` before any library sync.
-   - Keep a fallback path that uses `ListGames` when `Hello` is unsupported.
+   - Ignore peers that do not advertise the current protocol version.
 4. Library revisioning:
    - Store a monotonic `library_rev` locally and increment only after a
      successful index refresh completes.
@@ -180,8 +180,7 @@ Most scans become O(number of game dirs), with full recursion only when needed.
      incrementally update the cache.
    - Schedule a low-frequency full scan to reconcile missed watcher events.
 6. Announce updates:
-   - Replace `AnnounceGames` with `LibraryDelta` broadcasts keyed by
-     `library_rev`.
+   - Broadcast `LibraryDelta` updates keyed by `library_rev`.
    - Send `LibrarySummary` on new connections to seed the delta flow.
 7. File manifest caching:
    - Store per-game `manifest_hash` and only fetch details when changed.