performance: the song decides whether parts loop; no checkbox

[petersalomonsen]

This removes the performance checkbox. Whether a part loops is now decided by the song, and navigating between parts always works while it plays.

Why. The checkbox bundled four things:

1. waits active or inert;
2. whether a part can be jumped to by name;
3. the agent's fast path and stage tools;
4. an automatic switch of the agent's session.

Only the first is a real choice, and the song already states it: a part loops because it ends in waitForSignal.

What changes:

• Waits follow the song. Live playback always honours them. The new loopParts(false) makes every wait after it play straight through; it resets on each compile. An export (an OfflineAudioContext render) never waits, as before.
• Jumping to a part by name works whenever the song plays.
• The agent is on stage whenever the song plays and has parts:
  • part names and "next" are handled by the panel itself, without the model;
  • other text is a stage turn: stage prompt, low effort, where the playhead is.
• When stopped, the same text is an ordinary chat.
• The producer role also gets list_parts, go_to_part and send_signal.
• No automatic session switch. /new starts a fresh session, which is worth doing before a show.
• Removed: the checkbox and window.togglePerformanceMode.
• Docs: song-api.md has a new "Performance: looping parts and signals" section, including loopParts.

Tests:

• Sequencer: waits marked by loopParts(false) play through; the compiler marks the waits after it; each compile starts looping again. test-sequencer 16/16.
• Worklet spec, through the real audio worklet: parts loop by default, and loopParts(false) plays through. Chromium passes locally.
• Stage-agent e2e: stage behaviour while playing, ordinary chat when stopped. 2/2 locally.
• Roles, test-agent-tools 146/146; test-quickjs-sandbox 19/19; headless studio 3/3.

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