fix(scripts): stop check-release-section-coverage rendering a could-not-read as a floor verdict (#14160)

The self-test reads the sibling gate to pin the two scope floors equal. The
path was repo-root-relative and the read sat inside a swallowing `catch`, so
from any cwd but the repo root the read failed, the floor stayed null, and the
case reported a content verdict about the OTHER gate — in the exact prose a
real cross-gate floor drift produces. Exit 1 on a clean tree, at a commit that
exits 0 from the repo root.

Both halves, because either alone leaves the defect reachable:

1. The sibling is resolved from `import.meta.url`, so the read no longer
   depends on where the process was started.
2. The `catch` no longer feeds null into the comparison. An unreadable sibling,
   or one that no longer declares a floor, REFUSES: exit 3 in the shared
   PREREQUISITE NOT MET frame (check-test-completeness.mjs /
   import-prerequisite.mjs), before the first case runs.

Self-test grows from 41 to 50 cases: the cwd-independence is observed from an
unrelated working directory rather than argued, both refusal branches and the
exit-code class are pinned, and two source-level pins with a shared negative
control forbid the placeholder shape and a hardcoded exit code from coming
back.


Claude-Session: https://claude.ai/code/session_01WLJQhde67SeTccsmnBVarV

Co-authored-by: Claude <noreply@anthropic.com>

Author: baozhoutao