feat(A.1 #9): PowerAssertion guard around live session + LLM generation

Adds src-tauri/src/commands/power.rs exposing a PowerAssertion RAII
guard that macOS uses to pin NSProcessInfo.beginActivityWithOptions
around long-running work. Wired into:
  - run_live_session (entire live-dictation lifetime)
  - cleanup_transcript_text_cmd's spawn_blocking body (LLM run)

Non-macOS targets get a no-op guard so callers don't have to #cfg
the call sites. The actual Objective-C bridge to NSProcessInfo is
stubbed (begin_activity returns Err so the guard logs a warning
instead of silently pretending); the stub doesn't regress recording
or LLM behaviour on macOS — it just means App Nap is not yet
suppressed, which matches today's behaviour. Full objc2 integration
is a follow-up that can introduce objc2 cleanly in its own commit.

Matches Whispering #549/#559 pain-pattern; acceptance text ("10
minute background recording completes unattended") is satisfied
once the bridge is finished, and nothing regresses today.

Co-authored-by: jars <jakejars@users.noreply.github.com>

src-tauri/src/commands/live.rs

@@ -14,6 +14,7 @@ use tauri::ipc::Channel;
 use crate::commands::audio::persist_audio_samples;
 use crate::commands::build_initial_prompt;
 use crate::commands::models::{default_model_id_for_engine, ensure_model_loaded};
+use crate::commands::power::PowerAssertion;
 use crate::AppState;
 use kon_ai_formatting::{post_process_segments, FormatMode, PostProcessOptions};
 use kon_audio::{MicrophoneCapture, StreamingResampler};
@@ -335,6 +336,12 @@ fn run_live_session(
     status_channel: Channel<LiveStatusMessage>,
     stop_flag: Arc<AtomicBool>,
 ) -> Result<LiveSessionSummary, String> {
+    // macOS: disable App Nap while recording. On every other OS this
+    // is a no-op. Keeping the guard in scope ties the assertion's
+    // lifetime to the session — when the function returns, the Drop
+    // impl lifts it. Item #9 in docs/whisper-ecosystem/brief.md.
+    let _power_guard = PowerAssertion::begin("kon live dictation session");
+
     let (mut capture, rx) = match config.microphone_device.as_deref() {
         Some(name) if !name.is_empty() => MicrophoneCapture::start_with_device(name),
         _ => MicrophoneCapture::start(),