[quality] File or link the upstream V8 negative-count branch coverage report #550

Scope

Preserves the unimplemented upstream-report task previously folded into #507. The local documentation is tracked by #511; merging that PR does not complete this upstream task.

• Reproduce the negative-count BRDA behavior using the reduced example documented in docs(contributing): record the four negative-count BRDA branches as a v8 await artifact, not a gap #511, recording the current Node/Vitest/coverage-v8 versions.
• Find and link an existing matching vitest-dev/vitest report, or file a new report with the reproduction.
• Link the upstream report from the coverage note in docs/contributing.md.

Do not remove defensive code, add coverage ignores, or lower thresholds to hide the reporting artifact. Close only when the upstream reference and documentation link exist.