feat(kon): add transcription crate — transcribe-rs wrapper, model manager

- LocalEngine wraps transcribe-rs SpeechModel behind Kon's own abstraction
- load_parakeet() and load_whisper() factory functions
- Unified model manager: download with progress callback, check, list, path resolution
- Atomic downloads: .part suffix with rename on completion
- run_inference() encapsulates spawn_blocking threading
- Zero Tauri dependency in this crate (progress via callback, not events)
- transcribe-rs v0.3.2 with onnx + whisper-cpp features
- 4 tests passing, clippy clean

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/transcription/Cargo.toml

@@ -6,3 +6,13 @@ description = "Speech-to-text engine wrappers, model management, and inference c
 
 [dependencies]
 kon-core = { path = "../core" }
+
+# Unified STT engine (Parakeet via ONNX, Whisper via whisper.cpp)
+transcribe-rs = { version = "0.3", features = ["onnx", "whisper-cpp"] }
+
+# Async runtime for spawn_blocking
+tokio = { version = "1", features = ["rt", "sync"] }
+
+# Model downloads
+reqwest = { version = "0.12", features = ["stream"] }
+futures-util = "0.3"