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

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"))),