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/streaming/mod.rs

@@ -0,0 +1,72 @@
+//! Streaming primitives for live capture: VAD-gated chunking,
+//! agreement-based commit policy, and bounded buffer management.
+//!
+//! These types are tested at the unit level. Integration into
+//! `src-tauri/src/commands/live.rs` lands in follow-up commits so
+//! threshold tuning can be validated against real microphone captures
+//! rather than synthetic fixtures (brief items #21, #24, #25).
+
+pub mod rms_vad;
+
+pub use rms_vad::RmsVadChunker;
+
+/// A span of audio the VAD considers worth transcribing. `start_sample`
+/// is an absolute index into the stream the `VadChunker` has been fed
+/// since its last `reset`; `samples` is f32 PCM at the chunker's
+/// configured sample rate.
+#[derive(Debug, Clone)]
+pub struct VadChunk {
+    pub start_sample: u64,
+    pub samples: Vec<f32>,
+}
+
+/// A streaming VAD-gated chunker.
+///
+/// Implementations accumulate incoming samples, decide whether the
+/// current segment is speech using a score + hysteresis (brief item
+/// #21), and emit `VadChunk`s when a speech region ends — or when an
+/// in-progress speech region exceeds the configured max length so
+/// Whisper is not fed a 30-second monolith.
+///
+/// `push` returns any chunks ready to dispatch; typical usage is
+/// `for chunk in chunker.push(&samples) { dispatch(chunk); }` inside
+/// the capture loop.
+///
+/// `flush` is called at end-of-session to emit any in-flight speech as
+/// a final chunk (even if silence hasn't closed it).
+///
+/// `Send` because a chunker is owned by the live-session worker thread
+/// and moved into `spawn_blocking`.
+pub trait VadChunker: Send {
+    /// Feed new samples. Returns any chunks the chunker has decided to
+    /// emit as a result. An empty Vec means "still gathering".
+    fn push(&mut self, samples: &[f32]) -> Vec<VadChunk>;
+
+    /// End-of-session: emit any in-progress speech as a chunk even
+    /// though silence has not closed it. Returns `None` if there is
+    /// nothing buffered (or only sub-threshold samples).
+    fn flush(&mut self) -> Option<VadChunk>;
+
+    /// Drop accumulated state. Used between sessions on the same
+    /// chunker instance (or after a context-window reset from the
+    /// repetition detector — brief item #26).
+    fn reset(&mut self);
+
+    /// Absolute sample index of the next sample that will be fed via
+    /// `push`. Exposed so the commit policy (#24) can compute sample
+    /// offsets for its agreement window.
+    fn next_sample_index(&self) -> u64;
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn vad_chunker_trait_is_object_safe() {
+        // Compile-time witness: keep the trait dyn-compatible so the
+        // live-session worker can hold `Box<dyn VadChunker>` and swap
+        // between RMS and Silero backends at runtime.
+        let _: Option<Box<dyn VadChunker>> = None;
+    }
+}