Extract runtime resource element conversion

[chubes4]

Summary

• extract canvas, script, and template routing into one cohesive RuntimeResourceElementConverter
• move static-script classification, addressable JSON validation/serialization, runtime-island recording, and fallback capture out of HtmlTransformer
• remove five transformer helper methods and the now-unused latestScriptMetadata() state API
• reduce HtmlTransformer by 86 net lines (97 removed, 11 wiring/dispatch lines added)

Verification

• 15 direct runtime-resource assertions covering canvas, static JSON, executable script, template, metadata, islands, and handled-null behavior
• HTML transformer session-state contract
• 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 map the runtime-resource subsystem, implement the cohesive extraction, remove obsolete state, add direct tests, and run verification. The changes and evidence were reviewed and orchestrated by Chris Huber.