test(peer): cover installed-only rescan readiness

Add a focused local-library test for the case where a game starts with only a
committed `local/` install and later gains a root `version.ini` sentinel. The
single-game rescan now has coverage showing it promotes the summary from
LocalOnly to Ready while preserving the installed flag and reading the local
version.

This pins the cached-index transition called out in FOLLOW_UP_2.md without
touching scanner dispatch or broader monitor behavior.

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

Follow-up-Plan: FOLLOW_UP_2.md

crates/lanspread-peer/src/local_games.rs

@@ -716,6 +716,39 @@ mod tests {
         assert!(!scan.summaries.contains_key("non-catalog"));
     }
 
+    #[tokio::test]
+    async fn rescan_promotes_installed_only_game_to_ready_when_sentinel_appears() {
+        let temp = TempDir::new();
+        let catalog = HashSet::from(["game".to_string()]);
+        std::fs::create_dir_all(temp.path().join("game").join("local"))
+            .expect("local install dir should be created");
+
+        let first_scan = scan_local_library(temp.path(), &catalog)
+            .await
+            .expect("initial scan should succeed");
+        let local_only = first_scan
+            .summaries
+            .get("game")
+            .expect("installed-only game should be indexed");
+        assert!(!local_only.downloaded);
+        assert!(local_only.installed);
+        assert_eq!(local_only.availability, Availability::LocalOnly);
+
+        write_file(&temp.path().join("game").join("version.ini"), b"20250101");
+
+        let rescan = rescan_local_game(temp.path(), &catalog, "game")
+            .await
+            .expect("rescan should succeed");
+        let ready = rescan
+            .summaries
+            .get("game")
+            .expect("ready game should remain indexed");
+        assert!(ready.downloaded);
+        assert!(ready.installed);
+        assert_eq!(ready.eti_version.as_deref(), Some("20250101"));
+        assert_eq!(ready.availability, Availability::Ready);
+    }
+
     #[tokio::test]
     async fn local_download_available_gates_on_catalog_operation_and_sentinel() {
         let temp = TempDir::new();