feat: precache resilience — circuit breaker, forensic diag log, safe version check

- Circuit breaker: abort preload after 5 consecutive network failures
  (success or 404 resets streak) instead of grinding doomed retries
- localStorage diag ring buffer (survives cache wipes): preload-start/
  done/abort, manifest-unavailable, version events; shown in Settings
- checkDataVersion: no longer wipes IDB on transient read errors
  (DataCache.has tri-state probe); logs old value on real migrations
- loadManifest: null result no longer cached forever (per-session fallback lock-in)

src/core/data-cache.ts

@@ -93,6 +93,24 @@ export class DataCache {
     }
   }
 
+  /**
+   * Tri-state presence probe: true/false when known, null on read error.
+   * Unlike get(), lets callers distinguish "absent" from "unreadable".
+   */
+  static async has(key: string): Promise<boolean | null> {
+    try {
+      const db = await getDB();
+      return await new Promise<boolean | null>((resolve) => {
+        const tx = db.transaction(STORE_NAME, 'readonly');
+        const request = tx.objectStore(STORE_NAME).count(IDBKeyRange.only(key));
+        request.onsuccess = () => resolve(request.result > 0);
+        request.onerror = () => resolve(null);
+      });
+    } catch {
+      return null;
+    }
+  }
+
   /**
    * Store a value in the cache.
    */