The docs and the stallCandidates comment said the first candidate is
the span that blocked. The match is against the whole sample window, so
a slow span that ended before the stall can rank first. Describe the
candidates as leads, shortest first.

A line with candidates is about 1.5 KB, so a 1 MB stall log held about
700 per file and broke the documented last 1000 or more stalls. Raise
the cap to 1.5 MB, about 1,000 per file, and restore that promise.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>