Fix remaining 4 DOCX failures from issue #84

[developer0hye]

Summary

• Fix stack overflow on deeply nested tables (5000+ levels) in the docx-rs fork by adding a thread-local depth counter with RAII guard pattern (MAX_TABLE_DEPTH = 32)
• Add ReaderError::TableDepthExceeded error variant to distinguish depth-limit errors from other XML parse errors
• Propagate depth-exceeded errors from TableCell::read() while keeping tolerance for other table parse errors
• Add fixture tests for all 4 remaining DOCX files from issue upstream: docx-rs parse failures on valid DOCX files #84:
  • tdf171025_pageAfter.docx — ODT file with .docx extension (clean parse error)
  • tdf171038_pageAfter.docx — ODT file with .docx extension (clean parse error)
  • math-malformed_xml.docx — intentionally malformed XML (clean parse error)
  • deep-table-cell.docx — 5000+ nested tables (clean error via depth limit, no stack overflow)
• Remove deep-table-cell.docx from the bulk conversion DENYLIST

Test plan

• All 4 new fixture tests pass (smoke + structure for each)
• cargo test --workspace passes (all tests green)
• cargo fmt --all -- --check passes
• cargo clippy --workspace -- -D warnings passes
• docx-rs fork unit tests pass (271 tests, including 2 new depth-limit tests)

Related: #84

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