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>;

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(),
     }

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>>,

crates/core/src/recommendation.rs

@@ -1,7 +1,5 @@
 use crate::hardware::SystemProfile;
-use crate::model_registry::{
-    all_models, AccuracyTier, Engine, ModelEntry, SpeedTier,
-};
+use crate::model_registry::{all_models, AccuracyTier, Engine, ModelEntry, SpeedTier};
 use crate::types::Megabytes;
 
 /// A model's suitability score for a given system. Higher is better.
@@ -14,10 +12,7 @@ pub struct ScoredModel {
 
 /// Scores a single model against a system profile.
 /// Pure function, no side effects.
-pub fn score_model(
-    model: &'static ModelEntry,
-    profile: &SystemProfile,
-) -> Option<ScoredModel> {
+pub fn score_model(model: &'static ModelEntry, profile: &SystemProfile) -> Option<ScoredModel> {
     if model.ram_required > profile.ram {
         return None;
     }
@@ -41,9 +36,7 @@ pub fn score_model(
     if let Some(gpu) = &profile.gpu {
         let has_accel = match model.engine {
             Engine::Whisper => {
-                gpu.acceleration.metal
-                    || gpu.acceleration.vulkan
-                    || gpu.acceleration.cuda
+                gpu.acceleration.metal || gpu.acceleration.vulkan || gpu.acceleration.cuda
             }
             Engine::Parakeet | Engine::Moonshine => {
                 gpu.acceleration.cuda || gpu.acceleration.vulkan
@@ -55,8 +48,7 @@ pub fn score_model(
         }
     }
 
-    let headroom =
-        Megabytes(profile.ram.0.saturating_sub(model.ram_required.0));
+    let headroom = Megabytes(profile.ram.0.saturating_sub(model.ram_required.0));
     if headroom > Megabytes::from_gb(4.0) {
         score += 10.0;
     }
@@ -99,7 +91,7 @@ mod tests {
         SystemProfile {
             ram,
             cpu: CpuInfo {
-                core_count: 8,
+                logical_processors: 8,
                 brand: "Test CPU".into(),
             },
             gpu: None,
@@ -111,7 +103,7 @@ mod tests {
         SystemProfile {
             ram,
             cpu: CpuInfo {
-                core_count: 8,
+                logical_processors: 8,
                 brand: "Test CPU".into(),
             },
             gpu: Some(GpuInfo {
@@ -157,14 +149,12 @@ mod tests {
             .find(|m| m.engine == Engine::Parakeet)
             .expect("need a Parakeet model");
 
-        let gpu_score =
-            score_model(model, &profile_with_gpu(Megabytes(16384)))
-                .unwrap()
-                .score;
-        let cpu_score =
-            score_model(model, &profile_with_ram(Megabytes(16384)))
-                .unwrap()
-                .score;
+        let gpu_score = score_model(model, &profile_with_gpu(Megabytes(16384)))
+            .unwrap()
+            .score;
+        let cpu_score = score_model(model, &profile_with_ram(Megabytes(16384)))
+            .unwrap()
+            .score;
 
         assert!(gpu_score > cpu_score);
     }

crates/core/src/types.rs

@@ -168,6 +168,8 @@ pub struct TranscriptionOptions {
 
 /// Full provenance metadata for a transcript.
 /// Captures everything needed to reproduce the transcription.
+// TODO: Attach to Transcript once the store layer persists transcription provenance.
+#[allow(dead_code)]
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct TranscriptMetadata {
     pub engine: String,

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",