fix(core): use ReturnType<typeof setTimeout> for Deno Timeout compatibility

Author: SisyphusZheng

packages/core/src/island.ts

@@ -47,7 +47,7 @@ const log = createLogger('core');
 // Module-level store of active visibility strategy timeout IDs.
 // Used for test cleanup - tests can call _clearAllVisibilityTimeouts()
 // to prevent timer leaks.
-const _visibilityTimeouts: Set<number> = new Set();
+const _visibilityTimeouts = new Set<ReturnType<typeof setTimeout>>();
 
 /** Clear all active visibility strategy timeouts (for test cleanup). */
 export function _clearAllVisibilityTimeouts(): void {