Add full-site materialization parity fixture

[chubes4]

Summary

• Add a bounded full-site artifact parity fixture for php-transformer.
• Exercise generic materialization plan pages, routes, navigation links, menus, template part writes, writable text and base64 asset payloads, visual repair CSS, and source reports.
• Keep the fixture product-neutral with no SSI-specific expected keys.

Testing

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

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 + OpenCode
• Used for: Drafting the fixture, aligning expectations with the existing php-transformer contract, and running verification.