feat(A.3 #21): VadChunker trait + RMS backend (Silero deferred)

New crates/transcription/src/streaming/ module with:

- VadChunker trait: Send-bound, object-safe, push/flush/reset/
  next_sample_index. Same surface a future Silero backend will
  present, so live.rs wiring does not change when Silero drops in.
- VadChunk type: (start_sample: u64, samples: Vec<f32>) for
  commit-policy sample-offset bookkeeping in #24.
- RmsVadChunker: fallback backend the plan permits while the
  ort 2.0.0-rc.10 vs rc.12 ecosystem conflict blocks silero-vad-rust
  / voice_activity_detector. Tuned to match the existing
  evaluate_speech_gate behaviour (enter 0.003, exit 0.0014, 3-frame
  onset, 500 ms silence close, 2 s max chunk).

Key behavioural properties, each backed by a test:
- pure silence emits nothing
- samples between exit and enter thresholds never trigger onset
- a single loud frame does not start a chunk (sustained speech only)
- sustained speech followed by silence emits exactly one chunk
- hysteresis: a dip between enter and exit does not split a chunk
- max_chunk_samples caps continuous speech (Whisper never fed > 2 s)
- flush surfaces in-flight speech
- flush on an idle chunker emits nothing
- reset restores a clean state
- emitted chunk start_sample includes the onset buffer (Whisper sees
  the speech attack, not post-onset audio)

Open items tracked as follow-ups:
1. Silero backend via direct ort rc.12 bridge (Handy-style). Blocked
   on either ecosystem ort alignment or dedicated bridge session.
2. Integration into src-tauri/src/commands/live.rs. Deferred so
   threshold tuning can be validated against real microphone
   captures rather than synthetic constant-signal fixtures.

crates/transcription/src/lib.rs

@@ -1,6 +1,7 @@
 pub mod concurrency;
 pub mod local_engine;
 pub mod model_manager;
+pub mod streaming;
 pub mod transcriber;
 #[cfg(feature = "whisper")]
 pub mod whisper_rs_backend;
@@ -10,5 +11,6 @@ pub use local_engine::{load_parakeet, LocalEngine, SpeechModelAdapter, TimedTran
 #[cfg(feature = "whisper")]
 pub use local_engine::load_whisper;
 pub use model_manager::{download, is_downloaded, list_downloaded, model_dir, models_dir};
+pub use streaming::{RmsVadChunker, VadChunk, VadChunker};
 pub use transcriber::{Transcriber, TranscriberCapabilities};
 pub use transcribe_rs::SpeechModel;