Strengthen php-transformer contract fixtures

[chubes4]

Summary

• Rename the canonical HTML gap fixture to remove downstream converter acronym leakage from the fixture path/name.
• Add generic artifact report consistency assertions to parity coverage for materialization-plan totals and conversion-report summaries.
• Add a mixed site-artifact fixture covering canonical schema preservation, rejected unsafe input, HTML/block/Markdown pages, template parts, assets, visual repair CSS, and source report alignment.

Verification

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

Contract changes

• Strengthens test-enforced consistency between source_reports.materialization_plan.totals and source_reports.conversion_report.source_summary.
• Keeps artifact navigation, assets, routes, template parts, and visual repair reporting product-neutral.

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 + OpenCode
• Used for: Auditing php-transformer product-neutral boundaries, editing contract fixtures/tests, running verification, and drafting this PR description.