test(runtime): cover for-in visited levels across GC

[proggeramlug]

The runtime fix for #9869 is already on main, but the retained prototype levels had no parity regression that forces collection between recording a level and rebuilding the deferred shadow set. This fixture uses a keyed receiver, an empty middle prototype, and a keyed ancestor: middle-level key-array allocation evacuates the retained receiver, then the ancestor triggers the shadow rebuild and duplicate-key filter.

Validation:

• Exact fixture: 1/1 pass against Node 26.5.1
• Forced schedule completed 11 copying collections and moved 18,887 objects with from-space protection and evacuation verification enabled
• Test registration check passes: 259 files checked, no missing registrations
• scripts/run_lint_gates.sh: all 64 gates passed, 2 CI-only checks skipped locally
• Test-only change; no changeset or version bump required

Fixes #9869

Summary by CodeRabbit

• Tests
  • Added coverage for for-in iteration across a multi-level prototype chain.
  • Verifies that enumeration remains correct when garbage collection occurs during key collection.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Team

Run ID: 04a470a8-63b5-48fe-8e3d-d50739bbf91c

📥 Commits

Reviewing files that changed from the base of the PR and between 9869101 and 6f0f774.

📒 Files selected for processing (1)

• test-files/test_issue_9869_for_in_visited_levels_gc.ts

Included review availability: Your plan provides up to 8 included reviews per hour; 3 remain after this review.

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📝 Walkthrough

Walkthrough

Adds a parity test for issue #9869. The test enables forced moving GC, builds a three-level prototype chain with an empty middle level, iterates with for-in, and prints the collected keys.

Changes

for-in GC regression

Layer / File(s)	Summary
GC-sensitive for-in iteration test test-files/test_issue_9869_for_in_visited_levels_gc.ts	The test configures deterministic GC, builds the prototype chain, collects for-in keys, and prints the joined result after forced evacuation.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 6f0f7

This adds regression coverage for GC-sensitive prototype traversal during for-in iteration without changing runtime behavior. No current merge-readiness risk remains.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies a runtime regression test for for-in visited levels across garbage collection.
Description check	✅ Passed	The description explains the regression scenario, test design, linked issue, validation results, and test-only scope. It does not reproduce the template headings, but it contains the required informat…
Linked Issues check	✅ Passed	The test covers the deferred shadow-set scenario from [#9869] using a keyed receiver, an empty middle prototype, a keyed ancestor, forced moving collections, and duplicate-key filtering after prototyp…
Out of Scope Changes check	✅ Passed	The changes are limited to the requested parity regression fixture. No unrelated production, version, documentation, or configuration changes are reported.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 1…

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[proggeramlug]

Landed on main via merge train #9903. Validated as a tree: 64/64 lint gates, and perry-runtime/codegen/hir/stdlib all green (5,944 tests, 0 failures). Thanks!