Decompose inline SVG text tokens into native blocks

[chubes4]

Summary

• recognize classed inline text tokens containing materializable passive SVG artwork
• decompose supported text runs and SVGs into native paragraph and image children
• preserve source child order, including SVG-before-text and text-SVG-text tokens
• retain honest fallback for unsupported descendants or SVGs that require inline document context

Closes #592.

How to test

1. Check out this branch on a fresh clone.
2. Run composer --working-dir=php-transformer test:parity.
3. Confirm all 243 parity fixtures pass.
4. Run composer --working-dir=php-transformer test.
5. Confirm the PHP contracts, units, parity suite, and package-install proof pass.

Backwards compatibility

This intentionally changes generated block shape for the bounded recognized case: a classed <span> with supported inline text and passive materializable SVG siblings now becomes a core/group containing ordered core/paragraph and core/image blocks instead of a core/html fallback. Unsupported structures retain their prior fallback path. Consumers that inspect exact generated block arrays for this case should account for the new native shape; no extension paths or public PHP APIs change.

AI assistance

• AI assistance: Yes
• Tool(s): OpenAI gpt-5.6-sol via OpenCode
• Used for: Investigated representative SSI fallback evidence, implemented the generic native decomposition, reviewed source-order fidelity, and verified the transformer suites. Chris remains responsible for every line.