Fix dropped clicks and late screenshots in fast recording

Root cause of 'I clicked many times but only got two screenshots':
finishing/pausing a session called backend.stop(), which cancelled every
in-flight frame request to null. Clicks whose PNG had not finished
encoding yet were then dropped — only the first few survived.

Fixes:
- Stream backend now *drains* on stop: it stops accepting new requests but
  keeps the worker alive until frames already selected for queued clicks
  finish encoding. stop({ immediate: true }) keeps the old abandon-now
  behavior for an unhealthy worker.
- Two-stage worker reply: a fast 'frame-selected' ack pins the pairing and
  proves liveness; the slow PNG payload follows. A slow encode (seconds on
  software-rendered hosts) is no longer mistaken for a dead worker, which
  had been forcing the post-click fresh-shot fallback (late screenshots).
- Queued clicks carry their guide id and are stored even if the session
  ends while they wait in the queue.
- The tray gesture that stops a session is discarded by matching its
  recorded screen position, not a time window — a fast workflow click near
  the stop is no longer collateral damage. (Replaces the earlier grace
  window, which dropped whole bursts.)
- A click on a display with no ready stream resolves null so the caller
  fresh-shots the correct monitor instead of returning another screen.
- STEPFORGE_CAPTURE_LOG=1 prints one line per click decision; the
  second-instance handler now surfaces the running window instead of
  exiting silently.
- Self-test gains a fast-burst-then-finish scenario (8/8 saved) and the
  marker/coordinate checks remain at 0.00% offset.

Tests: 133 unit + all repo checks passing.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/renderer/capture-worker.js

@@ -150,6 +150,15 @@
       strict: cmd.strict !== false,
     });
     if (!frame) return reply({ ok: false, reason: 'no frame at or before the click' });
+    // Stage one: confirm the selection immediately. The encode below can
+    // take seconds on software-rendered hosts; without this ack the main
+    // process couldn't tell a slow encode from a dead worker.
+    send({
+      type: 'frame-selected',
+      requestId: cmd.requestId,
+      startedAt: frame.startedAt,
+      capturedAt: frame.capturedAt,
+    });
     try {
       const canvas = new OffscreenCanvas(frame.width, frame.height);
       canvas.getContext('2d').drawImage(frame.bitmap, 0, 0);