feat(outline): preserve headings across document formats

[ddebowczyk]

Summary

AnyDoc now preserves document-declared heading and outline structure across office, ebook, presentation, and spreadsheet formats.

User impact

• Restores Markdown headings for DOCX subtitles and style IDs, ODP slide/page names, RTF stylesheet names, EPUB3 navigation and EPUB2 NCX tables of contents, PPT/PPTX title and subtitle signals, spreadsheet sheet names, document metadata, OLE SummaryInformation titles, and RTF info titles.
• Adds Document.outline in Rust, Node, and Python with ordered level/text/anchor entries for navigation, indexing, chunking, and RAG workflows.
• Normalizes heading styles and avoids duplicate or empty metadata headings.
• Adds a guarded placeholder-free PPT/PPTX title heuristic with strict anti-false-positive conditions.
• Includes producer evidence for ODF and deliberately avoids an unverified text:p style-name heuristic.

Validation

• cargo fmt --all --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --locked (191 unit tests, robustness, snapshots)
• Node build and 13 tests
• Python binding tests: 8 passed on a supported Python 3.13 environment
• Built-in benchmark: 0 failures across the baseline corpus; the common 42-file mean moved from 9.79 ms to 10.71 ms (+0.92 ms, +9.4%), with the largest absolute per-file increase at +0.20 ms.

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