Harden php-transformer report consistency

[chubes4]

Summary

• Derive conversion report source-summary materialization counts from the canonical materialization plan totals when available.
• Add contract assertions that conversion_report and materialization_plan counts stay consistent for pages, assets, routes, navigation links, and menus.
• Extend the full-site materialization parity fixture and result-envelope docs with the generic count contract.

Tests

• composer test from php-transformer/
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 + OpenCode
• Used for: Inspecting the php-transformer contract surface, implementing the report-consistency hardening slice, adding behavior-level tests/docs, and running verification.