Echo Field integration

Echo Field is the submarine simulation game sndlab was originally built to support. Its audio architecture (see docs/audio-design.md in that repo) calls for procedurally synthesised sonar pings, ambient layers, and per-contact voices — exactly the shape sndlab’s DSL is designed for.

Pending the engine implementation and Echo Field’s audio module migration. The plan:

1. Echo Field declares sndlab-core as a dependency.
2. Audio patches live in crates/app/src/audio/patches/*.rhai (or embedded via include_str! for release builds).
3. At game startup, the audio engine evaluates the bundled scripts and registers patches by name.
4. Game events (SimEvent::TorpedoFired, …) trigger one-shots; the listener-state modulation surface drives ambient parameters.
5. The TUI sndlab session and the game share the same patch source — designing in sndlab and shipping in the game are the same act.