Add PocketTTS streaming support

[0xShug0]

Resolves #425

Adds PocketTTS streaming mode using the same acoustic generation path as offline mode, with streaming Mimi decode support.

Validation:

• Server streaming TTFT: 28.3 ms server-side, 28.6 ms client-observed on the warmed request.
• Offline path test: no regression; the existing long-short-long offline case produced byte-identical WAVs before and after the refactor.