Preserve compact styled select controls

[chubes4]

Summary

• Route static selects with resolved authored CSS presentation to a registered compact native blocks-engine/form-select companion block.
• Retain the existing readable group/list representation for unstyled selects and preserve runtime-targeted controls.
• Keep the legacy group/anchor boundary while moving authored select identity onto the native control.
• Avoid duplicate labels by rendering only the select in the companion block; the source label remains its editable sibling.
• Flatten the compatibility wrapper in frontend CSS so its native select receives authored grid/flex item sizing without changing valid block serialization.

Verification

• composer run test:canonical
• composer run test:parity (272 fixtures)
• Direct fixture20 combined input/select/flex candidate: Gutenberg validation passed 802/802 blocks.
• Fixture20 visual remains 1.53% (132,084/8,654,080 pixels); 48,498 pixels are classified restyle_geometry. The wrapper change did not move this residual, and the full-width submit remains covered by the separate flex-button candidate.

Fixture Integrity

No tracked fixture files were changed.

AI assistance

OpenAI gpt-5.6-terra via OpenCode implemented the transformer and companion-block changes, added contracts, and ran the listed verification. Chris Huber remains responsible for every line.