feat(A.2 #13): replace SpeechBackend enum with Transcriber trait

New crates/transcription/src/transcriber.rs defines a Transcriber
trait (Send supertrait for spawn_blocking travel) with
TranscriberCapabilities (sample_rate, channels, supports_initial_prompt).
TranscriberCapabilities.sample_rate is load-bearing for the upcoming
progressive WAV writer (#19).

Concrete impls:
- SpeechModelAdapter wraps Box<dyn transcribe_rs::SpeechModel + Send>
  for Parakeet (and any future transcribe-rs-backed engine).
- WhisperRsBackend moves its transcribe_sync body into the impl,
  widening the signature from &self to &mut self so per-call
  WhisperState can be created cleanly through the trait object.

LocalEngine now holds Box<dyn Transcriber + Send>; dispatch in
transcribe_sync collapses from a match to a direct call. Adds
LocalEngine::capabilities() for the WAV-writer.

Cargo feature flag "whisper" (default on) makes whisper-rs, num_cpus,
and the whole whisper_rs_backend module optional. cargo check
--no-default-features -p kon-transcription now builds without pulling
whisper-rs-sys — the escape hatch brief item #6 / #13 called for on
Windows / non-AVX2 / cloud-only builds. load_whisper is cfg-gated
behind the same feature.

src-tauri/src/commands/models.rs load_model_from_disk returns
Box<dyn Transcriber + Send> instead of SpeechBackend; caller chain
(ensure_model_loaded, prewarm_default_model) is unchanged.

transcriber_trait_is_object_safe test lands alongside the trait as a
compile-time witness against future Self-returning / generic-method
additions.

crates/transcription/src/transcriber.rs

@@ -0,0 +1,61 @@
+//! Engine-abstraction trait for speech-to-text backends.
+//!
+//! Replaces the previous `SpeechBackend` enum so new backends
+//! (Moonshine, whisper-rs forks, cloud ASR shims, Windows non-AVX2
+//! fallbacks) can drop in without adding a match arm in `LocalEngine`.
+//!
+//! Concrete implementers today: `SpeechModelAdapter` (wraps any
+//! `transcribe-rs` model, currently used for Parakeet) and — behind the
+//! `whisper` feature — `WhisperRsBackend` (direct whisper-rs, the only
+//! path that pipes `initial_prompt`).
+
+use kon_core::error::Result;
+use kon_core::types::{Segment, TranscriptionOptions};
+
+/// Static capabilities a `Transcriber` advertises to callers.
+///
+/// `sample_rate` is load-bearing for the progressive WAV writer (#19)
+/// which writes live capture samples to disk at the transcriber's
+/// native rate. `supports_initial_prompt` lets the Settings surface
+/// hide the initial-prompt field for backends that ignore it (Parakeet
+/// today).
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct TranscriberCapabilities {
+    pub sample_rate: u32,
+    pub channels: u16,
+    pub supports_initial_prompt: bool,
+}
+
+/// Unified interface for speech-to-text backends.
+///
+/// `Send` is a supertrait so `Box<dyn Transcriber + Send>` travels
+/// across `spawn_blocking` boundaries without a per-site bound. All
+/// inference is synchronous — async callers wrap a `tokio::spawn_blocking`
+/// around `transcribe_sync`.
+pub trait Transcriber: Send {
+    fn capabilities(&self) -> TranscriberCapabilities;
+
+    /// Synchronously transcribe 16 kHz mono f32 PCM (or whatever the
+    /// backend's `capabilities().sample_rate` declares). `&mut self` so
+    /// backends that keep per-call scratch state (whisper-rs's
+    /// `WhisperState`, Parakeet's decoder buffers) can mutate them
+    /// without interior-mutability gymnastics.
+    fn transcribe_sync(
+        &mut self,
+        samples: &[f32],
+        options: &TranscriptionOptions,
+    ) -> Result<Vec<Segment>>;
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn transcriber_trait_is_object_safe() {
+        // Compile-time witness: if the trait stops being object-safe
+        // (e.g. someone adds a generic method or a Self-returning
+        // method) this declaration fails to build. No runtime work.
+        let _: Option<Box<dyn Transcriber + Send>> = None;
+    }
+}