fix(runtime): unignore sanitizer and arena/genref regressions

[graydeon]

Summary

Two runtime regression areas remain behind ignored or placeholder coverage:

• sanitizer-backed runtime regression coverage for mutation-heavy map paths
• combined arena + generational-reference integration coverage

Why this matters

• these tests target memory-safety and lifetime-sensitive runtime behavior
• ignoring them leaves important runtime correctness questions unresolved
• the runtime should be able to support stronger regression checks in exactly these high-risk areas

Current behavior

• sanitizer-backed runtime regression coverage is still ignored due to unresolved leak/cleanup issues
• combined arena + generational-reference integration coverage is still ignored because the integration path needs debugging

Expected behavior

These regression suites should run normally and provide actionable protection for runtime memory-management changes.

Suggested scope

• resolve the remaining sanitizer-cleanliness failures in the affected runtime paths
• debug and stabilize the combined arena + generational-reference integration path
• unignore the affected tests
• document any remaining platform/tooling prerequisites for sanitizer-backed runs

Acceptance criteria

• the affected runtime regression tests can run without ignore markers in supported environments
• sanitizer-backed regressions pass cleanly where the toolchain is available
• arena + generational-reference integration has direct regression coverage