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/identity.rs

@@ -1,4 +1,4 @@
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 
 use uuid::Uuid;
 
@@ -7,8 +7,8 @@ const PEER_ID_FILE: &str = "peer_id";
 pub const FEATURE_LIBRARY_DELTA: &str = "library-delta-v1";
 pub const FEATURE_LIBRARY_SNAPSHOT: &str = "library-snapshot-v1";
 
-pub fn load_or_create_peer_id() -> eyre::Result<String> {
-    let path = peer_id_path();
+pub fn load_or_create_peer_id(state_dir: Option<&Path>) -> eyre::Result<String> {
+    let path = peer_id_path(state_dir);
     if let Ok(existing) = std::fs::read_to_string(&path) {
         let trimmed = existing.trim();
         if !trimmed.is_empty() {
@@ -31,7 +31,11 @@ pub fn default_features() -> Vec<String> {
     ]
 }
 
-fn peer_id_path() -> PathBuf {
+fn peer_id_path(state_dir: Option<&Path>) -> PathBuf {
+    if let Some(dir) = state_dir {
+        return dir.join(PEER_ID_FILE);
+    }
+
     if let Some(dir) = std::env::var_os("LANSPREAD_STATE_DIR") {
         return PathBuf::from(dir).join(PEER_ID_FILE);
     }