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/src/startup.rs

@@ -84,6 +84,7 @@ pub(crate) fn spawn_peer_runtime(
     game_dir: PathBuf,
     unpacker: Arc<dyn Unpacker>,
     catalog: Arc<RwLock<std::collections::HashSet<String>>>,
+    enable_mdns: bool,
 ) -> PeerRuntimeHandle {
     let shutdown = CancellationToken::new();
     let task_tracker = TaskTracker::new();
@@ -102,6 +103,7 @@ pub(crate) fn spawn_peer_runtime(
             runtime_shutdown.clone(),
             runtime_tracker.clone(),
             catalog,
+            enable_mdns,
         )
         .await
         {
@@ -125,7 +127,9 @@ pub(crate) fn spawn_peer_runtime(
 
 pub(crate) fn spawn_startup_services(ctx: &Ctx, tx_notify_ui: &UnboundedSender<PeerEvent>) {
     spawn_quic_server(ctx, tx_notify_ui);
-    spawn_peer_discovery_service(ctx, tx_notify_ui);
+    if ctx.enable_mdns {
+        spawn_peer_discovery_service(ctx, tx_notify_ui);
+    }
     spawn_peer_liveness_service(ctx, tx_notify_ui);
     spawn_local_library_monitor(ctx, tx_notify_ui);
 }