Revert non-PHP SSI parity package changes

[chubes4]

Summary

• Reverts only the package-side changes from Preserve source nav anchor CSS on core navigation #511 under packages/blocks-engine/**.
• Preserves the PHP transformer changes from Preserve source nav anchor CSS on core navigation #511 and all SSI-relevant transformer work.
• Leaves CHANGELOG.md untouched.

Why

SSI fixture-matrix parity work consumes blocks-engine/php-transformer through SSI_FIXTURE_MATRIX_BLOCKS_ENGINE_PHP_TRANSFORMER_PATH. Changes under packages/blocks-engine/** are not part of that path and should not ship as SSI parity fixes.

Verification

• Confirmed diff is limited to:
  • packages/blocks-engine/src/__tests__/theme-source-assets-parity.test.ts
  • packages/blocks-engine/src/theme/index.ts
  • packages/blocks-engine/src/theme/source-assets.ts
• Ran git diff --check.

AI assistance

• AI assistance: Yes
• Tool(s): OpenAI GPT-5.5 via OpenCode
• Used for: Audited the interrupted PR wave, identified the package-side SSI parity contamination, and prepared the narrow corrective revert PR.

[chubes4]

Closing this corrective revert after re-checking the source diff from #511. The TypeScript changes are not just SSI fixture noise: they add buildNavigationAnchorCompatCss() to the package source-assets pipeline, export it, and cover it with focused parity tests for replaying source nav anchor selectors through core/navigation wrapper markup. The PHP artifact compiler carries the SSI-facing implementation, so SSI fixture reliability still primarily depends on the PHP transformer, but reverting the package-side implementation would remove a legitimate product improvement for the JS/package transformer surface.

Decision: keep the #511 TypeScript package change; do not merge this revert.

AI assistance

• AI assistance: Yes
• Tool(s): OpenAI GPT-5.5 via OpenCode
• Used for: Re-inspected the Preserve source nav anchor CSS on core navigation #511 diff and current Revert non-PHP SSI parity package changes #517 status, then drafted this source-evidence decision comment.