Make HTML compilation run-scoped

[chubes4]

Summary

• turn HtmlTransformer into a stateless 40-line public facade over immutable runtime and analysis-cache inputs
• run each document through a fresh HtmlCompilation with one stable session and collaborator graph
• preserve public constants, emitted-block contracts, fallback reduction, diagnostic attribution, and shared analysis caching
• redirect intentional white-box tests to the run-scoped compilation boundary

This is the lifetime prerequisite for replacing lazy transformer callbacks with typed session-bound collaborators. The implementation body is copied into the run-scoped class without conversion-policy changes.

Verification

• composer test (292 parity fixtures and package-install proof)
• 385 HTML fixtures: identical serialized-block hashes, 0 exceptions
• reused-transformer state isolation and shared-analysis-cache contracts
• Homeboy PR architecture audit: 0 introduced findings
• git diff --check

AI assistance

GPT-5.6 Sol via OpenCode analyzed the session-lifetime coupling, implemented the run-scoped boundary, updated internal architecture contracts, and ran the full verification suite under human direction.