Compose specialized element converters

[chubes4]

Summary

• compose details, button, and SVG conversion through one ordered specialized-element registry
• replace three dedicated transformer fields with one registry field
• preserve details → button → SVG precedence and first-handled semantics

Verification

• direct registry, details, button, and SVG converter contracts
• composer validate --strict
• composer test
• 292 PHP transformer parity fixtures
• exact-base CSS-attached corpus: 383 documents, 87 fixtures, 82 with CSS, 0 throws
• base/candidate SHA-256: 6f0fd03d6eaa5b9683919e19a0a44eb652f7517373b91c4de3880ac9055ab04f

Part of #242.

AI assistance

OpenAI GPT-5.6-sol via OpenCode was used to inspect the contiguous converter dispatch, compose it through the ordered registry, and run verification. The changes and evidence were reviewed and orchestrated by Chris Huber.