fix(fs): enforce write limits on chmod copy-on-write

[chaliy]

Summary

• Call check_write_limits() before copy-on-write in chmod() to prevent bypassing limits
• Ensure parent directory exists in upper before writing (same pattern as other CoW paths)

Test plan

• test_chmod_cow_enforces_write_limits - chmod on lower file near limit is rejected
• All existing overlay tests pass

Closes #417