feat(converters): add OpenDocument (ODT/ODS/ODP) support

[buleboy (cyberspace-cs)]

Summary

Adds OpenDocument format support: .odt (text), .ods (spreadsheet), .odp (presentation), plus the flat XML variants .fodt/.fods/.fodp.

Why

OpenDocument is a widely used office format (LibreOffice, Google Docs export), but it is currently unsupported. Per the project's contribution guidance ("be conservative about new formats because of dependencies"), this converter is implemented with zero new dependencies — ODF is just ZIP + XML, so it uses the standard library zipfile and the project's existing defusedxml.

What changed

• converters/_odf_converter.py (~600 lines): converter registration, ODF metadata extraction, text/spreadsheet/presentation rendering.
• tests/test_odf_converter.py: 54 test cases + 2 real fixtures.
• Converter registration in converters/_converters.py, README format table, and docs updated.

Design notes

1. table:number-columns-repeated="16384" is a compression artifact, not data. Table width is taken from the last cell with actual content, not the last grid cell; repeated non-empty cells are expanded with MAX_REPEATED_CELLS/MAX_COLUMNS caps to prevent memory blowups.
2. Whitespace is markup: text:s / text:tab are elements that must survive whitespace collapsing — otherwise a<text:s/>b is indistinguishable from ab.
3. Cells are containers: multi-paragraph cells are joined with <br>; table:covered-table-cell is kept to preserve column alignment; body type is determined by office:text/office:spreadsheet/office:presentation rather than the extension.

Security

• Uses defusedxml (not the stdlib XML parser) because content.xml comes from untrusted files — entity expansion and external entities are allowed under xml.dom.minidom.
• 64 MiB uncompressed size cap.

Tests

• Full suite: 1061 passed.
• Baseline on a clean HEAD worktree: 989 passed / 15 failed. The only new failures are test_convert_http_uri[test_vector15/16], which fail because the new fixtures are not yet on GitHub main; those tests are explicitly skipped via skip_remote in CI.
• black --check clean.

Dependencies

No new dependencies (stdlib zipfile + existing defusedxml).

Checklist

• New format adds no new dependencies
• Tests added (54 cases)
• README / docs updated
• black --check clean

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