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/context.rs

@@ -39,6 +39,7 @@ pub struct Ctx {
     pub unpacker: Arc<dyn Unpacker>,
     pub catalog: Arc<RwLock<HashSet<String>>>,
     pub peer_id: Arc<String>,
+    pub enable_mdns: bool,
     pub shutdown: CancellationToken,
     pub task_tracker: TaskTracker,
 }
@@ -54,6 +55,7 @@ pub struct PeerCtx {
     pub peer_game_db: Arc<RwLock<PeerGameDB>>,
     pub catalog: Arc<RwLock<HashSet<String>>>,
     pub peer_id: Arc<String>,
+    pub enable_mdns: bool,
     pub tx_notify_ui: tokio::sync::mpsc::UnboundedSender<PeerEvent>,
     pub shutdown: CancellationToken,
     pub task_tracker: TaskTracker,
@@ -72,6 +74,7 @@ impl std::fmt::Debug for PeerCtx {
 
 impl Ctx {
     /// Creates a new context with the given peer game database.
+    #[allow(clippy::too_many_arguments)]
     pub fn new(
         peer_game_db: Arc<RwLock<PeerGameDB>>,
         peer_id: String,
@@ -80,6 +83,7 @@ impl Ctx {
         shutdown: CancellationToken,
         task_tracker: TaskTracker,
         catalog: Arc<RwLock<HashSet<String>>>,
+        enable_mdns: bool,
     ) -> Self {
         Self {
             game_dir: Arc::new(RwLock::new(game_dir)),
@@ -92,6 +96,7 @@ impl Ctx {
             unpacker,
             catalog,
             peer_id: Arc::new(peer_id),
+            enable_mdns,
             shutdown,
             task_tracker,
         }
@@ -111,6 +116,7 @@ impl Ctx {
             peer_game_db: self.peer_game_db.clone(),
             catalog: self.catalog.clone(),
             peer_id: self.peer_id.clone(),
+            enable_mdns: self.enable_mdns,
             tx_notify_ui,
             shutdown: self.shutdown.clone(),
             task_tracker: self.task_tracker.clone(),