fix: show pre-merge and post-merge coverage per PR

[helly25]

Show one coverage result per PR, replacing pre-merge coverage only when a report tests its exact merge commit. Remove standalone main rows and show one result per phase in the secondary view while preserving every archived report URL.

AG;DR

• Adapt Show coverage by PR with pre-merge and post-merge results xff#884 to the existing Rust LCOV publisher and Python generator. Refresh a minimal PR registry and attribute main reports by exact merge SHA; never substitute unrelated main or aggregation results for constituent PRs.
• Select the newest run and numeric retry per phase, always preferring post-merge in the overview. Handle late arrivals, post-merge-only PRs, closure/reopening, and immutable snapshots.
• Depends on fix: refresh coverage on PR merge, close, and reopen #137, now merged; this branch is synchronized with main at 605f6d3 and targets main directly. No outstanding parent merge dependency.
• Python standard library only; no workflow, build-tool, or dependency changes. No new source files.
• Validation on pushed head 6044ded against main 605f6d3: all 58 Python tests and applicable pre-commit hooks passed. CI run 35465572580 passed all jobs including the final done gate. The exact-merge regression fails against the previous implementation. Parent fix: refresh coverage on PR merge, close, and reopen #137 main CI 35465500675 and coverage publication 35465595330 both succeeded.
• Post-merge coverage requires an available report for the exact merge commit. Otherwise pre-merge remains visible. Publishing changes take effect after merge through the existing serialized publisher.