[FEATURE] Mods: document the 4-concurrent-clip limit of $.audio.play

The mod API docs for $.audio.play say "clips are not queued, so two calls play together", but the engine refuses a fifth concurrent clip per plugin (refused: 4 plays are going at once). The docs don't mention this, so a mod that plays short overlapping clips (e.g. a piano) silently loses notes when the user plays fast.

Suggestions

• Document the limit in the type docs for $.audio.play.
• Expose it (e.g. $.audio.limits), or make it configurable.
• Optionally offer a voice-stealing mode that stops the oldest clip instead of refusing the new one.

Environment

• Claude Code 2.1.296, macOS (desktop app, Code tab)
• Workaround used in the mod: keep at most 4 clips going and abort the oldest (via PlayOptions.signal) before playing a new one.