fix(test): register two dark GC string tests in the repsel corpus

[proggeramlug]

Summary

check_test_registration is red on main: #8439 and #8443 each added a
test_gap_gc_* fixture to test-files/ without registering it in
test-parity/gc_repsel_corpus.txt, so scripts/gc_repsel_matrix.sh (gc-stress,
gc-moving-witnesses, gc-ptr-shape-off-witness) never executed either.

- DARK TEST test-files/test_gap_gc_string_copy_source_rooting.ts
- DARK TEST test-files/test_gap_gc_string_repeat_reentrant_count.ts

A GC regression test that exists on disk but never runs is worse than no test: it reads as
coverage in review and provides none. That is the exact failure mode CLAUDE.md calls out —
"the gate runs but its subject never did".

I caught this while batch-validating the string series and fixed it locally, but the fix
lived in my stack rather than in the PRs, so it did not travel through the squash merges.

Validation

• python3 scripts/check_test_registration.py — exit 0
• python3 scripts/check_test_registration.py --self-test — exit 0

Summary by CodeRabbit

• Tests

  • Added two previously omitted garbage-collection stress tests to the representation-selection test suite.
  • These tests are now included in the automated GC test matrix.

• Documentation

  • Added a changelog entry documenting the newly registered tests.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: f0d22f90-536d-43c3-b657-24b2b140fd55

📥 Commits

Reviewing files that changed from the base of the PR and between 5c1544d and f5b397f.

📒 Files selected for processing (2)

• changelog.d/8461-register-dark-gc-tests.md
• test-parity/gc_repsel_corpus.txt

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📝 Walkthrough

Walkthrough

The change registers two previously omitted GC stress tests in the representation-selection corpus and adds a changelog entry describing the corrected test-matrix coverage.

Changes

GC corpus registration

Layer / File(s)	Summary
Register and document GC tests test-parity/gc_repsel_corpus.txt, changelog.d/8461-register-dark-gc-tests.md	Adds test_gap_gc_string_copy_source_rooting and test_gap_gc_string_repeat_reentrant_count to the corpus. Documents their moving-GC scenarios and the coverage correction.

Estimated code review effort: 1 (Trivial) | ~3 minutes

Suggested reviewers: jdalton

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/register-dark-gc-tests

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