Preserve CSS-significant semantic geometry

[chubes4]

Summary

• preserve independently styled inline semantic leaves when paragraph/RichText lowering would lose selector identity or flex/grid item geometry
• retain selector-compatible inline hooks in valid Gutenberg RichText, including dynamic pseudo-state projection
• preserve responsive SVG width and intrinsic viewBox aspect ratio using Gutenberg-canonical image save markup
• strengthen source-anchor selector projection for native Button blocks

Refs #624. This is a measured intermediate improvement for Automattic/static-site-importer#489; it does not claim exact visual parity or make 15-saas eligible for solved-fixture promotion.

Results

• post-Preserve author selector semantics across native block save shapes #623 aligned mismatch: 24.89%
• this PR aligned mismatch: 18.34%
• raw mismatch: 25.48% -> 18.66%
• editor validity: 291/291 native blocks
• core/html: 0
• global alignment offset: 0px, 0px

Supplementary Lab evidence: run summary. The exact-parity gate still fails because lower-page attribution is truncated by the current 160-element visual-compare capture.

How to test

1. Check out this branch in a fresh clone.
2. Run cd php-transformer && composer test.
3. Run php tests/unit/author-selector-semantics.php; confirm all 21 assertions pass.
4. Run php tests/contract/run.php.
5. Run php tests/parity/run.php; confirm all 244 fixtures pass.
6. Run Static Site Importer's documented fixture-matrix wrapper against this checkout with the authored saas tag and exact visual gating.
7. Confirm all 291 blocks are editor-valid and native, no core/html blocks are emitted, and the aligned mismatch is 18.34%.

Backwards compatibility

This changes generated block markup and projected author CSS for selector-addressed inline leaves, responsive materialized SVGs, and promoted Button controls. It does not change the public PHP API. Consumers comparing generated markup byte-for-byte may observe intentional output changes.

AI assistance

• AI assistance: Yes
• Tool(s): OpenAI gpt-5.6-sol via OpenCode
• Used for: Diagnosed browser evidence, implemented and reviewed generic transformer primitives, added deterministic coverage, and prepared the PR.