refactor(core): remove redundant as casts using window.d.ts declarations

window.d.ts already declares __timelines, __player, __playerReady,
and __renderReady on the global Window interface. The casts in
init.ts and init.test.ts were re-asserting the same types.

- Add __hfRuntimeTeardown to window.d.ts (used 6x in init.ts)
- Remove runtimeWindow cast variable from init.ts — use window directly
- Remove all (window as Window & { __player?: ... }).__player casts
  from init.test.ts — window.__player is already typed as PlayerAPI
- Remove all (window as Window & { __timelines?: ... }).__timelines
  casts from init.test.ts — window.__timelines is already typed
- Remove (window as Window & { __playerReady/renderReady }}) casts
  from init.ts — already declared globally

packages/core/src/runtime/window.d.ts

@@ -31,6 +31,7 @@ declare global {
     __clipManifest?: RuntimeTimelineMessage;
     __playerReady?: boolean;
     __renderReady?: boolean;
+    __hfRuntimeTeardown?: (() => void) | null;
     __HF_PARITY_MODE?: boolean;
     __HF_FPS?: number;
     __HF_MAX_DURATION_SEC?: number;