feat: add revision-aware semantic PDF comparison - #547

Summary

• add bounded semantic PDF comparison across visual, textual, structural, metadata, security, and serialization domains
• attribute changes to incremental revisions and report equivalent/unreachable objects
• normalize serialization and XMP timestamp noise with shared resource budgets
• add qpdf/poppler differential coverage and require the external tools in CI
• resolve the Kripteia fixture false positive and related lint findings

Validation

• cargo test -p oxidize-pdf
• cargo test -p oxidize-pdf semantic_comparison --lib
• cargo test -p oxidize-pdf --test semantic_comparison_differential
• focused clippy with -D warnings
• Kripteia security: no issues
• Kripteia test quality: 95/100

Closes #543