fix: address PR #57 CodeRabbit feedback

- Acquire AniList post-watch in-flight lock before async gating to prevent duplicate writes
- Isolate manual watched mark result from AniList post-watch callback failures
- Report known-word cache clears as mutations during immediate append when state existed
- Add regression tests for each fix

src/core/services/ipc.test.ts

@@ -326,6 +326,38 @@ test('registerIpcHandlers runs AniList update after manual mark watched succeeds
   assert.deepEqual(calls, ['mark', 'anilist']);
 });
 
+test('registerIpcHandlers isolates AniList update failures after manual mark watched succeeds', async () => {
+  const { registrar, handlers } = createFakeIpcRegistrar();
+  const calls: string[] = [];
+  const originalWarn = console.warn;
+  console.warn = () => undefined;
+
+  try {
+    registerIpcHandlers(
+      createRegisterIpcDeps({
+        immersionTracker: createFakeImmersionTracker({
+          markActiveVideoWatched: async () => {
+            calls.push('mark');
+            return true;
+          },
+        }),
+        runAnilistPostWatchUpdateOnManualMark: async () => {
+          calls.push('anilist');
+          throw new Error('post-watch failed');
+        },
+      }),
+      registrar,
+    );
+
+    const result = await handlers.handle.get(IPC_CHANNELS.command.markActiveVideoWatched)?.({});
+
+    assert.equal(result, true);
+    assert.deepEqual(calls, ['mark', 'anilist']);
+  } finally {
+    console.warn = originalWarn;
+  }
+});
+
 test('registerIpcHandlers skips AniList update when manual mark watched has no active session', async () => {
   const { registrar, handlers } = createFakeIpcRegistrar();
   const calls: string[] = [];

src/core/services/ipc.ts

@@ -390,7 +390,14 @@ export function registerIpcHandlers(deps: IpcServiceDeps, ipc: IpcMainRegistrar
   ipc.handle(IPC_CHANNELS.command.markActiveVideoWatched, async () => {
     const marked = (await deps.immersionTracker?.markActiveVideoWatched()) ?? false;
     if (marked) {
-      await deps.runAnilistPostWatchUpdateOnManualMark?.();
+      try {
+        await deps.runAnilistPostWatchUpdateOnManualMark?.();
+      } catch (error) {
+        console.warn(
+          'Failed to run AniList post-watch update after manual watched mark:',
+          (error as Error).message,
+        );
+      }
     }
     return marked;
   });