Fix native player state when playback is blocked

[happykawayigt]

Summary

• keep NativePlayer paused until the native play event confirms playback
• handle rejected HTMLMediaElement.play() promises without leaving stale state
• add a regression test for autoplay-policy rejection

Root cause

NativePlayer.play() set its internal paused flag to false immediately, even when the browser rejected the underlying playback promise. No native pause event follows a rejected play attempt, so transcript polling could continue indefinitely against a player that never started.

Validation

• npm run build
• npm test -- --runInBand (75 tests passed)
• git diff --check

Fixes #268