Support canonical php-transformer site artifact schema

[chubes4]

Summary

• Exposes ArtifactCompiler::INPUT_SCHEMA as the canonical blocks-engine/php-transformer/site-artifact/v1 site artifact schema.
• Adds a contract assertion that canonical site artifact inputs are accepted and preserved in source_reports.artifact.original_schema.
• Updates Static Site Importer migration adapter docs/tests to consume the canonical transformer result envelope instead of BAC-shaped result/input schemas.

Verification

• composer validate
• composer test
• git diff --check

Downstream impact

• Static Site Importer can switch away from block-artifact-compiler/website-artifact/v1 and use blocks-engine/php-transformer/site-artifact/v1 as the canonical input schema.
• BAC references remain only in compatibility-wrapper guidance, not in canonical SSI adapter schema expectations.
• No tag or release included.

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Auditing schema contracts, editing php-transformer docs/tests/contracts, running verification, and drafting this PR description.