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/Cargo.toml

@@ -5,3 +5,8 @@ edition = "2021"
 description = "Core types, constants, traits, hardware detection, and model registry for Kon"
 
 [dependencies]
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+thiserror = "2"
+sysinfo = "0.35"
+async-trait = "0.1"