Emit stable evidence for core/html transformer fallbacks

[chubes4]

Summary

• add a versioned generic taxonomy and bounded evidence envelope for every PHP transformer core/html emission
• include source selector/path, structural source-subtree digest/snippet, emitted block/content digests, and totals/truncation
• merge evidence across artifact documents and project it into conversion reports and WordPress site-plan quality/reporting
• add contracts for deterministic, payload-safe transformer and site-plan evidence

Closes #836.

Verification

composer test

AI assistance

OpenAI GPT-5.6 Terra via OpenCode was used to inspect the transformer/evidence pipeline, implement the bounded fallback-evidence contract, and run the PHP test suite. Chris Huber remains responsible for the change.