Introduce Docxodus (a modernized .NET 8.0 fork of Open-XML-PowerTools with
better move detection) as an alternative engine alongside XmlPowerToolsEngine.
- Extract BaseEngine class with shared binary extraction and subprocess logic
- XmlPowerToolsEngine and DocxodusEngine are thin subclasses setting 3 constants
- Add Docxodus as a git submodule at docxodus/
- Refactor build_differ.py into reusable build_engine() function (also fixes
 missing win-arm64 compression)
- Update CI workflow for submodules and .NET SDK
- Add integration tests and parametrized contract tests for both engines

Rewrite README to prominently feature Docxodus as the recommended comparison
engine, with a link back to the Docxodus repo. Reorganize sections around the
dual-engine architecture and add a quick example.

Thread WmlComparerSettings options from Python kwargs through CLI
flags to the Docxodus C# binary. Supports detail_threshold,
case_insensitive, detect_moves, simplify_move_markup,
move_similarity_threshold, move_minimum_word_count,
detect_format_changes, conflate_spaces, and date_time.
- Extract _build_command() in BaseEngine, override in DocxodusEngine
- Add input validation for thresholds and word count
- Update Docxodus CLI to parse --flags (backward compat with legacy format)
- Rebuild all platform binaries with new flag support
- Add 13 new tests (integration, validation, unit)
- Update README with Comparison Settings section

Run tests across 3 OSes x 3 Python versions on push to main
and on pull requests. Includes package build verification.

Add global.json to pin the .NET SDK to 8.0.x, preventing CI runners
with .NET 10 pre-installed from using the wrong compiler (which breaks
Docxodus due to List<T>.Reverse() vs LINQ Reverse() resolution).
Also fix build_differ.py run_command() to raise on non-zero exit codes
instead of silently continuing past build failures.