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/hardware.rs

@@ -13,7 +13,7 @@ pub struct SystemProfile {
 
 #[derive(Debug, Clone)]
 pub struct CpuInfo {
-    pub core_count: usize,
+    pub logical_processors: usize,
     pub brand: String,
 }
 
@@ -49,16 +49,16 @@ pub enum Os {
     Android,
 }
 
-pub fn probe_ram() -> Megabytes {
-    let sys = System::new_all();
+/// Probes RAM from a shared `System` instance.
+fn probe_ram_from(sys: &System) -> Megabytes {
     let total_bytes = sys.total_memory();
     Megabytes(total_bytes / (1024 * 1024))
 }
 
-pub fn probe_cpu() -> CpuInfo {
-    let sys = System::new_all();
+/// Probes CPU info from a shared `System` instance.
+fn probe_cpu_from(sys: &System) -> CpuInfo {
     CpuInfo {
-        core_count: sys.cpus().len(),
+        logical_processors: sys.cpus().len(),
         brand: sys
             .cpus()
             .first()
@@ -75,32 +75,30 @@ pub fn probe_gpu() -> Option<GpuInfo> {
 
 pub fn probe_os() -> Os {
     #[cfg(target_os = "windows")]
-    {
-        Os::Windows
-    }
+    return Os::Windows;
     #[cfg(target_os = "linux")]
-    {
-        Os::Linux
-    }
+    return Os::Linux;
     #[cfg(target_os = "macos")]
-    {
-        Os::MacOs
-    }
+    return Os::MacOs;
     #[cfg(target_os = "ios")]
-    {
-        Os::Ios
-    }
+    return Os::Ios;
     #[cfg(target_os = "android")]
-    {
-        Os::Android
-    }
+    return Os::Android;
+
+    // Fallback for unsupported targets — treat as Linux since
+    // most exotic/embedded targets are Unix-like.
+    #[allow(unreachable_code)]
+    Os::Linux
 }
 
-/// Composes the individual probes. No logic here — just assembly.
+/// Composes the individual probes using a single `System` snapshot.
+/// `System::new_all()` is expensive — calling it once rather than
+/// per-probe avoids redundant OS queries.
 pub fn probe_system() -> SystemProfile {
+    let sys = System::new_all();
     SystemProfile {
-        ram: probe_ram(),
-        cpu: probe_cpu(),
+        ram: probe_ram_from(&sys),
+        cpu: probe_cpu_from(&sys),
         gpu: probe_gpu(),
         os: probe_os(),
     }