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/providers.rs

@@ -0,0 +1,38 @@
+use std::sync::Arc;
+
+use async_trait::async_trait;
+
+use crate::error::Result;
+use crate::types::{AudioSamples, EngineName, Transcript, TranscriptionOptions};
+
+/// Any speech-to-text engine implements this trait.
+/// Base types know nothing about their derivatives.
+#[async_trait]
+pub trait SpeechToText: Send + Sync {
+    async fn transcribe(
+        &self,
+        audio: AudioSamples,
+        options: &TranscriptionOptions,
+    ) -> Result<Transcript>;
+
+    fn name(&self) -> &EngineName;
+
+    fn is_available(&self) -> bool;
+}
+
+/// Any text post-processor implements this trait.
+#[async_trait]
+pub trait TextProcessor: Send + Sync {
+    async fn process(&self, text: &str, instruction: &str) -> Result<String>;
+
+    fn name(&self) -> &EngineName;
+
+    fn is_available(&self) -> bool;
+}
+
+/// Holds the active provider instances. Constructed at startup,
+/// rebuilt when user changes provider in settings.
+pub struct ProviderRegistry {
+    pub stt: Arc<dyn SpeechToText>,
+    pub text: Option<Arc<dyn TextProcessor>>,
+}