Opt-in setting so sounds follow the iPhone silent switch (and the Android
ringer mode) while still mixing with other audio. Off by default, so
existing apps behave as before.

- iOS uses the ambient category (audioplayers rejects respectSilence
  together with mixWithOthers there); Android uses the ringtone usage type
  with no audio focus. Existing players get the new context too.
- Web and desktop ignore it.
- Tests for the audio context of each platform, README and CHANGELOG.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>