perf(meeting): cache sysinfo System for the meeting-detection poller

`detect_meeting_processes` is called every 15 s when meeting-auto-capture
is enabled. The previous `list_running_process_names` allocated a fresh
`sysinfo::System` per call and walked /proc cold; on a busy host
(~300 processes) that's ~50–100 ms of work, every poll, forever.

Add `kon_core::process_watch::ProcessLister`, a thin wrapper around a
long-lived `System` whose process table is refreshed in place. The Tauri
host holds one behind a `Mutex<ProcessLister>` in a new `MeetingState`
managed at app setup. The free `list_running_process_names` is kept as a
convenience that constructs a fresh `ProcessLister` per call — its only
remaining caller is the existing smoke test.

- ProcessLister + Default in crates/core/src/process_watch.rs.
- MeetingState in src-tauri/src/commands/meeting.rs; the command takes
  it via `tauri::State` and locks for the duration of the snapshot.
- src-tauri/src/lib.rs registers MeetingState alongside the other
  managed states.

https://claude.ai/code/session_0189xUb6ie6t9qHkzatGZ9Rb

crates/core/src/process_watch.rs

@@ -8,19 +8,57 @@
 
 use sysinfo::{ProcessRefreshKind, ProcessesToUpdate, RefreshKind, System};
 
-/// Snapshot the current process list's executable/command names. Lowercased
-/// for case-insensitive pattern matching.
-pub fn list_running_process_names() -> Vec<String> {
-    let mut system = System::new_with_specifics(
+/// Reusable wrapper around a `sysinfo::System` whose process table is
+/// refreshed in place on every poll, instead of allocating a fresh one.
+///
+/// On a busy host (~300 processes), `System::new_with_specifics` followed by
+/// `refresh_processes` walks `/proc` cold and costs ~50–100 ms; reusing the
+/// same instance reuses sysinfo's per-process bookkeeping so subsequent
+/// refreshes are dominated by diffing rather than allocation. The Tauri
+/// host holds one of these behind a `Mutex` for the meeting-detection
+/// command to call every 15 s.
+pub struct ProcessLister {
+    system: System,
+}
+
+impl Default for ProcessLister {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl ProcessLister {
+    pub fn new() -> Self {
+        Self {
+            system: System::new_with_specifics(
                 RefreshKind::nothing().with_processes(ProcessRefreshKind::nothing()),
-    );
-    system.refresh_processes(ProcessesToUpdate::All, true);
-    system
+            ),
+        }
+    }
+
+    /// Refresh the process table in place and return the current
+    /// lowercased executable names.
+    pub fn snapshot(&mut self) -> Vec<String> {
+        self.system
+            .refresh_processes(ProcessesToUpdate::All, true);
+        self.system
             .processes()
             .values()
             .map(|process| process.name().to_string_lossy().to_lowercase())
             .collect()
     }
+}
+
+/// Snapshot the current process list's executable/command names. Lowercased
+/// for case-insensitive pattern matching.
+///
+/// Convenience wrapper that allocates a fresh `ProcessLister` per call.
+/// Hot paths (the meeting-detection poller) should hold a long-lived
+/// `ProcessLister` and call `snapshot()` directly to avoid the per-call
+/// allocation of `System`'s internal bookkeeping.
+pub fn list_running_process_names() -> Vec<String> {
+    ProcessLister::new().snapshot()
+}
 
 /// Match a snapshot of process names against case-insensitive substring
 /// `patterns`. Returns the set of patterns that matched at least once, in

src-tauri/src/commands/meeting.rs

@@ -5,13 +5,46 @@
 //! that reminds the user to start recording with their hotkey. We do not
 //! start recording from this signal — the user decides.
 
-use kon_core::process_watch;
+use std::sync::Mutex;
+
+use kon_core::process_watch::{self, ProcessLister};
+
+/// Tauri-managed state for the meeting poller. Holds a long-lived
+/// `ProcessLister` so each poll refreshes the existing `sysinfo::System`
+/// in place instead of allocating a fresh one — the previous implementation
+/// rebuilt the process table from scratch every 15 s.
+pub struct MeetingState {
+    lister: Mutex<ProcessLister>,
+}
+
+impl MeetingState {
+    pub fn new() -> Self {
+        Self {
+            lister: Mutex::new(ProcessLister::new()),
+        }
+    }
+}
+
+impl Default for MeetingState {
+    fn default() -> Self {
+        Self::new()
+    }
+}
 
 #[tauri::command]
-pub fn detect_meeting_processes(patterns: Vec<String>) -> Result<Vec<String>, String> {
+pub fn detect_meeting_processes(
+    state: tauri::State<'_, MeetingState>,
+    patterns: Vec<String>,
+) -> Result<Vec<String>, String> {
     if patterns.is_empty() {
         return Ok(Vec::new());
     }
-    let processes = process_watch::list_running_process_names();
+    let processes = {
+        let mut lister = state
+            .lister
+            .lock()
+            .map_err(|e| format!("MeetingState lister poisoned: {e}"))?;
+        lister.snapshot()
+    };
     Ok(process_watch::match_meeting_patterns(&processes, &patterns))
 }

src-tauri/src/lib.rs

@@ -250,6 +250,7 @@ pub fn run() {
             app.manage(commands::audio::NativeCaptureState::new());
             app.manage(commands::live::LiveTranscriptionState::default());
             app.manage(commands::tts::TtsState::new());
+            app.manage(commands::meeting::MeetingState::new());
 
             app.manage(AppState {
                 whisper_engine: Arc::new(LocalEngine::new(EngineName::new("whisper"))),