feat(kon): add core crate — types, traits, hardware, model registry, recommendation

- Value objects: ModelId, EngineName, Megabytes, AudioSamples, Segment, Transcript
- KonError enum with thiserror
- Constants centralised: audio pipeline, VAD, RAM thresholds, inference threading
- SpeechToText and TextProcessor provider traits with ProviderRegistry
- Unified model registry (Whisper tiny/base/small/medium + Parakeet CTC int8)
- Hardware detection: probe_ram, probe_cpu, probe_gpu (stub), probe_os
- Recommendation engine: score_model (pure function), rank_recommendations (sorted)
- 5 tests passing, clippy clean

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

crates/core/src/lib.rs

@@ -1,2 +1,13 @@
-// kon-core: Foundation types, constants, provider traits, hardware detection,
-// model registry, and recommendation engine.
+pub mod constants;
+pub mod error;
+pub mod hardware;
+pub mod model_registry;
+pub mod providers;
+pub mod recommendation;
+pub mod types;
+
+pub use error::{KonError, Result};
+pub use types::{
+    AudioSamples, DownloadProgress, EngineName, Megabytes, ModelId, Segment,
+    Transcript, TranscriptionOptions,
+};