Preserve authored classes and flex layout on anchor buttons

[chubes4]

Summary

• Remove authored anchor-root classes from both core/button surfaces.
• Retain generated provenance markers on the canonical button wrapper, projecting matched anchor-root selectors onto marker > .wp-block-button__link.
• Bridge direct flex-child anchors through core/buttons > core/button > link: reset synthetic wrapper gap/margin, retain column stretch width through both wrappers, and keep paint/content on the canonical link.
• Keep direct child selectors on preserved RichText descendants, including flex: 1, and add real Gutenberg parse/save validity coverage.

Verification

• composer run test:canonical
• composer run test:parity (272 fixtures)
• node node_modules/vitest/vitest.mjs run src/wp/__tests__/button-save-validity.test.ts
• Exact fixture20 stack: Preserve compact styled select controls #831 c84fc531 + Preserve authored static controls as compact native blocks #832 5174a285 + this PR 417ba565; SSI df7bb346; WP Codebox 78bebd6c. Gutenberg validation passed 802/802 blocks with zero editor warnings.
• Fixture20 aligned visual mismatch improved from 2.50% to 1.53% (132,084/8,654,080 pixels; 2.37% raw). The seasonal/button clusters are absent from remaining regions; remaining differences are unrelated restyle_geometry and one 1,041-pixel missing/extra element region. Evidence: /var/folders/lr/c_cmmt7s0592m4njz99v5yb40000gn/T/opencode/fixture20-831-832-833-flex-bridge-retry/.

Fixture Integrity

No tracked fixture files changed.

Depends on #831. Closes #830.

AI assistance

OpenAI gpt-5.6-terra via OpenCode implemented the generic flex-wrapper bridge, added contracts, and ran the listed verification. Chris Huber remains responsible for every line.