List immutable audio-environment presets

Returns the backend-owned, versioned catalogue used by Voice Library, agent voice configuration, preview auditions and Simulation Lab. The catalogue is a pure local read: it contacts no TTS provider, makes no carrier call, changes no agent setting and returns no audio bytes. off is explicit. An agent's outgoing selection always pins preset id, asset version and volume; Simulation Lab's separate caller-input condition pins the same id/version with noise_level. Callers must not invent a frontend catalogue, silently substitute a retired version, or pass output ambience to STT.