fix: align tombstone key validation

[b2m9]

Summary

• validate tombstones against enumerable own string and symbol keys
• ignore unsafe delta keys consistently with ordinary object folds
• add regressions for symbol-bearing and unsafe-key tombstones

Testing

• npm run check

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Summary by cubic

Aligns tombstone key validation with merge behavior so we only check enumerable own keys and consistently ignore unsafe keys. Prevents false rejections when symbol-key patch data is present.

• Bug Fixes
  • Validate tombstones using enumerableOwnKeys (includes symbol keys, excludes inherited).
  • Ignore UNSAFE_KEYS on tombstones, matching ordinary delta object handling.
  • Add tests for symbol-keyed fields and unsafe keys on tombstones.

^{Written for commit 1861cb8. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}