fix(peer): sanitize legacy library index migration

The selected game root could supply an unbounded legacy library index whose
revision was copied verbatim into app-owned state. A revision of `u64::MAX`
made every later checked revision advance fail even after the original root was
removed.

Read only a bounded regular, non-link file whose identity stays stable across
the read. Deserialize it before publication, preserve its cached game data, and
reset its stale revision authority to zero. Existing app-owned state still wins
without reading the legacy source, and invalid input remains in place for
recovery.

Test Plan:
- `just test` -- passed outside the sandbox; 494 peer tests and all workspace
  targets passed.
- The initial sandboxed `just test` reached an unrelated Unix-socket permission
  denial, then passed unchanged with the required socket permission.
- `git diff --cached --check` -- passed.

crates/lanspread-peer/src/local_games.rs

@@ -191,6 +191,14 @@ struct LibraryIndex {
     games: HashMap<String, GameIndexEntry>,
 }
 
+/// Converts a legacy selected-root cache into app-owned state without carrying
+/// its stale revision authority across the trust boundary.
+pub(crate) fn normalize_migrated_library_index(bytes: &[u8]) -> serde_json::Result<Vec<u8>> {
+    let mut index: LibraryIndex = serde_json::from_slice(bytes)?;
+    index.revision = 0;
+    serde_json::to_vec(&index)
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 struct GameIndexEntry {
     summary: LocalGameSummary,
@@ -1043,6 +1051,29 @@ mod tests {
         }
     }
 
+    #[test]
+    fn migrated_library_index_preserves_games_but_resets_revision_authority() {
+        let original = test_library_index(u64::MAX, "game", 77);
+        let bytes = serde_json::to_vec(&original).expect("legacy index should serialize");
+
+        let normalized =
+            normalize_migrated_library_index(&bytes).expect("legacy index should normalize");
+        let migrated: LibraryIndex =
+            serde_json::from_slice(&normalized).expect("normalized index should deserialize");
+
+        assert_eq!(migrated.revision, 0);
+        assert_eq!(migrated.games.len(), 1);
+        assert_eq!(
+            migrated
+                .games
+                .get("game")
+                .expect("game should be preserved")
+                .summary
+                .size,
+            77,
+        );
+    }
+
     #[test]
     fn legacy_fingerprint_defaults_download_recovery_to_false() {
         let fingerprint: GameFingerprint = serde_json::from_value(serde_json::json!({