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/lib.rs

@@ -1,9 +1,14 @@
 pub mod concurrency;
 pub mod local_engine;
 pub mod model_manager;
+pub mod transcriber;
+#[cfg(feature = "whisper")]
 pub mod whisper_rs_backend;
 
 pub use concurrency::run_inference;
-pub use local_engine::{load_parakeet, load_whisper, LocalEngine, SpeechBackend, TimedTranscript};
+pub use local_engine::{load_parakeet, LocalEngine, SpeechModelAdapter, TimedTranscript};
+#[cfg(feature = "whisper")]
+pub use local_engine::load_whisper;
 pub use model_manager::{download, is_downloaded, list_downloaded, model_dir, models_dir};
+pub use transcriber::{Transcriber, TranscriberCapabilities};
 pub use transcribe_rs::SpeechModel;