fix(ext-api/tests): partial TypeScript cleanup in BC tests

- Prefix unused variable with _ (cleanups → _cleanups)
- Use globalThis.setInterval for portable timer type
- Prefix unused nodeType param with _

Note: Pre-existing TS errors in other files remain (lazy-serialize, bc-04, etc.)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/extension-api-v2/__tests__/bc-02.migration.test.ts

@@ -131,7 +131,7 @@ describe('BC.02 migration — node lifecycle: teardown', () => {
       const v1Ticks = vi.fn()
       const v2Ticks = vi.fn()
 
-      let v2Handle: number | undefined
+      let v2Handle: ReturnType<typeof globalThis.setInterval> | undefined
 
       // v1 pattern: manual tracking
       const v1Handle = setInterval(v1Ticks, 100)

src/extension-api-v2/__tests__/bc-02.v2.test.ts

@@ -182,7 +182,7 @@ describe('BC.02 v2 contract — node lifecycle: teardown', () => {
     it('interval cleared in onScopeDispose does not fire after unmount', () => {
       vi.useFakeTimers()
       const intervalCallback = vi.fn()
-      let handle: number | undefined
+      let handle: ReturnType<typeof globalThis.setInterval> | undefined
 
       const { unmount } = mountNode(() => {
         handle = setInterval(intervalCallback, 100)