Improve sidebar form column detection - #553

Summary

• Ignore HTML comments when evaluating candidate column containers.
• Treat a form-tagged sibling as the content pane when paired with a sidebar child.
• Add contract coverage for a commented sidebar-plus-form layout converting to native columns.

Verification

• php tests/contract/run.php
• php tests/unit/button-signal-classifier.php
• Direct Artist contact transform emits one core/columns block.
• SSI Artist focused surface matrix rerun: run id 9091e2c3-efeb-42d6-a48e-76245048b8f5, still failing on remaining visual/editor/runtime blockers.

AI assistance

• AI assistance: Yes
• Tool(s): GPT-5.5 via OpenCode
• Used for: Investigation, implementation, focused verification, and PR drafting.