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>

app/renderer/capture-worker.js

@@ -147,6 +147,7 @@
     await sampleFrame(state);
     const frame = StepForgeClickFrames.selectFrameForClick(state.ring.frames(), {
       clickAt: cmd.clickAt,
+      leadMs: cmd.leadMs || 0,
       mode: 'fullscreen',
       strict: cmd.strict !== false,
     });