refactor: reduce global window.__frameScript coupling

[ubugeeei]

Finding

Several core APIs publish imperative hooks through window.__frameScript. This makes renderer automation convenient but spreads mutable global state across frame, image, audio waveform, and animation modules.

Evidence

• src/lib/frame.tsx, src/lib/image.tsx, src/lib/audio-waveform.ts, and src/lib/animation.ts write to window.__frameScript.

Acceptance criteria

• Define a typed FrameScript bridge interface in one module.
• Centralize registration/cleanup behavior.
• Preserve the current render automation API.

[ubugeeei]

対応済みです。修正は PR #1 / commit 9cf4498 に反映済みです。