Warm the frame recorder before hiding the window at recording start

The first screenshot of a session was late while every later one was fine.
Cause: on 'Start recording' the window hid first and the capture backend
started warming up after — creating worker, getUserMedia, first frame takes
~1s. A click in that gap found no buffered frame and took the post-click
fresh shot.

armRecording() now warms the recorder while the window is still visible and
only hides once frames are buffering (with a brief post-hide settle so the
first frame shows the user's screen, not the dismissed app window). Verified
end to end with a new self-test scenario that clicks 250ms after start: the
first click is now served a pre-click frame instead of a post-click shot.

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

docs/CHANGELOG.md

@@ -30,6 +30,12 @@ Keep-a-Changelog conventions; versions follow semver.
     cadence was tightened to 50ms so a frame near that target always exists.
     The lead is a preference, not a gate: selection falls back to the newest
     frame still before the click, so it never forces a post-click screenshot.
+  - The frame recorder now warms up *before* the window hides at recording
+    start, instead of after. Previously the first click of a session could
+    beat the ~1s warmup and fall back to a post-click shot — "the first
+    screenshot is late" — while every later click was fine. Now frames are
+    buffering by the time the window tucks away, so the first click is
+    served a pre-click frame like the rest.
 
 ### Fixed