fix(kon): harden core crate — shared System instance, accurate CPU field, serialisable errors

- Share single System::new_all() in probe_system() instead of calling it twice
- Rename CpuInfo::core_count to logical_processors (sys.cpus().len() returns logical, not physical)
- Add fallback arm to probe_os() for unsupported cfg targets
- Add serde::Serialize to KonError for structured frontend error reporting
- Annotate dead code (ProviderRegistry, TranscriptMetadata) with #[allow(dead_code)] + TODO comments
- Update downstream references in recommendation tests and tauri hardware command

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

crates/core/src/providers.rs

@@ -32,6 +32,8 @@ pub trait TextProcessor: Send + Sync {
 
 /// Holds the active provider instances. Constructed at startup,
 /// rebuilt when user changes provider in settings.
+// TODO: Wire into Tauri app state once multi-engine switching is implemented.
+#[allow(dead_code)]
 pub struct ProviderRegistry {
     pub stt: Arc<dyn SpeechToText>,
     pub text: Option<Arc<dyn TextProcessor>>,