Never take a post-click screenshot when a pre-click frame exists

The remaining 'captured slightly after the click' reports came from the
fresh-shot fallback, which grabs the screen when the click is processed
(after it). The previous lead change made that fallback *more* likely: a
frame now had to be >=120ms before the click to qualify, so on machines
where the capture stream can't always keep a frame that old buffered, more
clicks fell through to the post-click shot.

Make the click-lead a two-tier preference instead of a hard gate in
selectFrameForClick:
1. newest frame captured at least leadMs before the click (ideal margin), else
2. newest frame captured before the click at all.
Only when no pre-click frame exists does the caller fresh-shot. leadMs is
threaded through the stream backend to the worker so both selection paths
agree. Verified end to end: frames land ~120-170ms before each click,
markers stay at 0.00%, and the 8-click burst still saves all 8.

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

docs/CHANGELOG.md

@@ -24,10 +24,12 @@ Keep-a-Changelog conventions; versions follow semver.
   - Physical→DIP coordinate conversion is multi-monitor and scale-factor
     aware (`screen.screenToDipPoint` on Windows, display-geometry math
     elsewhere), fixing marker drift on displays scaled away from 100%.
-  - A configurable click-lead (`capture.clickLeadMs`, default 120ms) targets
-    the screen just before each click so the saved step shows what the user
-    was about to act on, not the click's onset; the stream sampling cadence
-    was tightened to 50ms so a frame near that target always exists.
+  - A configurable click-lead (`capture.clickLeadMs`, default 120ms) prefers
+    a frame captured a little before each click so the saved step shows what
+    the user was about to act on, not the click's onset; the stream sampling
+    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.
 
 ### Fixed