fix(coverage): a line with two ERROR nodes reports the same range twice

[CaptainMittens]

scripts/setup-windows.ps1 has 326 lines. Its parse-coverage report reads:

113-113,113-113,245-327

Line 113 is named twice. Found while building the CI coverage gate in #1941; not fixed there.

What happens

Line 113 carries two separate ERROR nodes, at columns 25-29 and 31-32. cbm_error_regions_push pushes a range for each one. A line range is advice — "read these lines" — and saying it a second time adds nothing.

Why it matters beyond the noise

Both copies count against CBM_MAX_ERROR_REGIONS. A file with many multi-error lines can therefore be clipped, and report ,+<N>, while carrying fewer distinct lines than the cap allows.

Suggested fix

Merge a region that overlaps the range already open instead of appending a repeat. The walk visits children in source order, so a region that starts at or before the open range's end really does overlap it. The merge has to run before the cap check, or a merged region is miscounted as a dropped one.

Related: #963, #1941. Sibling finding: the past-EOF end line in the same range string.