Moss tts realtime

[voxmenthe]

Realtime Runtime: Session Lifecycle + Streaming + Chunk/Overlap Decoding

This PR adds the dedicated MOSS realtime runtime path, including explicit session lifecycle controls, incremental text-delta streaming, and chunked audio decoding with overlap crossfade.

Scope (Included)

This PR includes:

• mlx_audio/tts/models/moss_tts_realtime/**
• mlx_audio/tts/tests/test_moss_tts_realtime_runtime.py
• tests/tts/models/moss_tts_realtime/test_realtime_regressions.py
• examples/moss_tts_realtime_multiturn_agent.py
• examples/moss_tts_realtime_text_deltas.py
• mlx_audio/tts/models/qwen3_tts/speech_tokenizer.py

Primary behavior in scope:

• Dedicated realtime inferencer/session runtime (separate from non-realtime flow).
• Explicit session lifecycle semantics: reset_turn -> text ingestion -> end_text -> drain.
• Text-delta streaming bridge with incremental emission.
• Bounded pending-frame buffer (backpressure/fail-fast behavior).
• Chunked decode with overlap crossfade controls (chunk_frames, overlap_frames).
• Realtime sampling controls including repetition-window behavior.
• Realtime examples aligned to the runtime lifecycle contract.

Scope (Intentionally Excluded)

This PR does not include:

• Non-realtime MOSS runtime behavior (mlx_audio/tts/models/moss_tts/**).
• Generic TTS CLI gateway refactors in mlx_audio/tts/generate.py.
• Server/API request schema or endpoint behavior in mlx_audio/server.py.
• Broad docs/readme expansion outside the realtime examples listed above.

Validation

uv run pytest -q \
  mlx_audio/tts/tests/test_moss_tts_realtime_runtime.py \
  tests/tts/models/moss_tts_realtime/test_realtime_regressions.py

Additional targeted checks run during review:

uv run pytest -q mlx_audio/tts/tests/test_moss_tts_realtime_runtime.py -k \
  'bridge_and_direct_token_paths_produce_equal_chunk_lengths or \
   overlap_crossfade_non_final_decode_path_is_mlx_safe or \
   overlap_crossfade_shrinking_flush_is_sample_conservative'

Result: 3 passed (targeted overlap/bridge checks).

Manual Smoke Notes

• Realtime examples were checked against current RealtimeSession API semantics and lifecycle method signatures.
• Short chunk/overlap decoder smoke showed overlap reduces boundary discontinuity versus no-overlap:
  • overlap=2 boundary jumps: [0.4583, 0.2083]
  • overlap=0 boundary jumps: [0.9583, 0.9583]

Reviewer Guide

Suggested review order:

1. Runtime tests first (test_moss_tts_realtime_runtime.py, realtime regressions).
2. inference.py session lifecycle and decoder/backpressure/crossfade paths.
3. model.py realtime runtime wiring and generation flow.
4. Realtime examples (moss_tts_realtime_text_deltas.py, moss_tts_realtime_multiturn_agent.py) for user-facing contract parity.

[lucasnewman]

Closing for now. We'll reopen if work on #501 is resumed.