fix: respect frozen review scope in triage collection

[eminos]

Summary

When active_triage_issue_ids freezes a review cycle, the collector still includes older concerns and later review arrivals. Reflect then rejects a ledger that fully covers the frozen cycle. Use the existing coverage_open_ids helper for required review coverage, preserving live history, mechanical backlog and new-arrival context.

 review_issues
-  all open review issues
+  open review issues in coverage_open_ids(plan, state)

Evidence

• Before: Two regression tests fail: a frozen two-issue cycle collects four issues, and reflect rejects its complete two-issue ledger.
  After: All 17 focused tests pass on current upstream, including eight new regression cases for complete and incomplete frozen ledgers, unrelated open issues, retained history, and complete unfrozen coverage.

Merge Danger

Door: Two-way

Blast Radius: Triage

The collector uses the same scope helper as snapshot and downstream planning gates. Reflect's parser and accounting rules remain unchanged.