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/error.rs

@@ -1,8 +1,14 @@
 use std::path::PathBuf;
 
+use serde::Serialize;
+
 use crate::types::ModelId;
 
-#[derive(Debug, thiserror::Error)]
+/// Structured error type for Kon.
+///
+/// Implements `Serialize` so errors can be sent to the frontend as
+/// structured JSON rather than opaque strings.
+#[derive(Debug, thiserror::Error, Serialize)]
 pub enum KonError {
     #[error("model not found: {0}")]
     ModelNotFound(ModelId),
@@ -32,10 +38,23 @@ pub enum KonError {
     StorageError(String),
 
     #[error("io error: {0}")]
-    Io(#[from] std::io::Error),
+    Io(
+        #[from]
+        #[serde(serialize_with = "serialize_io_error")]
+        std::io::Error,
+    ),
 
     #[error("{0}")]
     Other(String),
 }
 
+/// Serialises `std::io::Error` as its display string, since it does
+/// not implement `Serialize` natively.
+fn serialize_io_error<S: serde::Serializer>(
+    err: &std::io::Error,
+    s: S,
+) -> std::result::Result<S::Ok, S::Error> {
+    s.serialize_str(&err.to_string())
+}
+
 pub type Result<T> = std::result::Result<T, KonError>;