feat(pdf): add to_markdown_pages for per-page extraction

[HaoChiBao]

Summary

• Expose MarkdownPage plus to_markdown_pages / to_markdown_pages_bytes wrapping pdf-inspector's per-page extraction (page boundaries, OCR flags)
• Keep PDF to_document unsupported; leave existing single-blob to_markdown / to_markdown_bytes behavior unchanged
• Document the new API in the README Rust/PDF sections

Closes #62

Why not to_document for PDF

PDFs intentionally bypass the document model (pdf-inspector emits Markdown directly). A thin per-page API is the low-risk path from the issue and does not race the OCR work in #61.

Test plan

• cargo fmt --all
• cargo clippy --locked --all-targets -- -D warnings
• cargo test --locked (fixture tests/fixtures/pdf/text.pdf)
• Optional follow-up: Node/Python/wasm bindings for the new API

---

Summary by cubic

Adds per-page Markdown extraction for PDFs so callers can keep page boundaries and detect OCR needs. Previously we only returned a single Markdown blob and errored on fully scanned PDFs; now to_markdown_pages/to_markdown_pages_bytes return one item per page with a needs_ocr flag, while single-blob APIs remain unchanged.

• Exposes MarkdownPage { page, markdown, needs_ocr, ocr_reason } and exports it from the crate.

• Keeps to_markdown/to_markdown_bytes behavior: still one string; still errors on image-only PDFs and logs when some pages need OCR.

• Adds to_markdown_pages/to_markdown_pages_bytes: never fail solely due to OCR; pages that need OCR have empty markdown and needs_ocr = true and may include ocr_reason. Logs a warn when any pages need OCR.

• Leaves to_document unsupported for Format::Pdf; updates the Unsupported message to mention to_markdown_pages.

• Updates README and adds a basic per-page test.

• Migration (optional): If you need page boundaries or to route OCR per page, switch PDF callers to to_markdown_pages/to_markdown_pages_bytes. Otherwise, keep using to_markdown/to_markdown_bytes.

^{Written for commit 5d0c50f. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 4 files

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