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>

src-tauri/src/commands/hardware.rs

@@ -31,7 +31,7 @@ pub fn probe_system() -> Result<SystemInfo, String> {
     Ok(SystemInfo {
         ram_mb: profile.ram.0,
         cpu_brand: profile.cpu.brand,
-        cpu_cores: profile.cpu.core_count,
+        cpu_cores: profile.cpu.logical_processors,
         os: match profile.os {
             Os::Windows => "Windows",
             Os::Linux => "Linux",