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>

tests/unit/capture.test.js

@@ -798,8 +798,8 @@ test('click frames come from the stream backend when it is active', async () =>
 
   assert.equal(result.ok, true);
   assert.deepEqual(added, ['stream-frame']);
-  assert.deepEqual(requests, [{ clickPos: { x: 10, y: 10 }, clickAt, strict: true }],
-    'the worker receives the hook-time click timestamp and strictness');
+  assert.deepEqual(requests, [{ clickPos: { x: 10, y: 10 }, clickAt, strict: true, leadMs: 0 }],
+    'the worker receives the hook-time click timestamp, strictness, and lead');
 });
 
 test('a stream backend with no qualifying frame falls through to the fresh-shot path', async () => {