feat(pdf): add document page composition APIs

[messense]

Summary

• add Rust-native page range, selection, insertion option, and insertion result types
• add document page insert/copy/duplicate/move/delete/select/reload helpers
• add page label and xref convenience APIs

Stack

Base: main

Validation

• git log --oneline main..codex/api-parity-page-composition
• cargo test --lib -- --skip document_writer::test::test_writer_ocr on the top of the dependent stack
• cargo test was also run on the stack tip; it only failed at the existing OCR test because local Tesseract data is missing (./eng.traineddata).