Refactor PHP shell landmark policy

[chubes4]

Summary

• Extract shared PHP shell/landmark detection into ShellLandmarkPolicy.
• Reuse it from HtmlTransformer, SemanticParityReporter, and ArtifactCompiler template-part area detection.
• Add unit coverage for global shell vs content-local landmark boundaries.

Tests

• composer test:unit
• composer test

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Inspected PHP transformer/materialization code, implemented the shell landmark policy extraction, added tests, and ran verification.