refactor(voice): own the Pocket TTS engine in Berd

[johnmatthewtennant]

Summary

Moves Pocket TTS inference from the Buzz dependency into a Berd-owned berd-voice crate, so the engine can evolve without Buzz releases.

The crate contains the April ONNX runtime, tokenizer and chunking logic, and reference-voice loading used by Berd’s native voice path. Model installation metadata remains in the existing Berd installer instead of being duplicated in the crate.

Reviewer-reproducible examples

1. Check out this PR and launch Berd with just dev.
2. In Voice settings, install the native voice models and select a Pocket voice.
3. Start a voice conversation and prompt for a spoken response.
4. Confirm Berd synthesizes and plays the response through the native voice path, and that stopping playback still interrupts it.