fix(settings): disable torch compilation by default on Windows

[art22s]

Resolves #3956

Summary

• Disable torch.compile() by default on Windows, where TorchInductor requires an MSVC compiler that may not be installed.
• Preserve explicit opt-in through DOCLING_INFERENCE_COMPILE_TORCH_MODELS=true and per-engine compile_model=True.
• Add a regression test covering the Windows default and the environment-variable override.

Testing

• python3 -m py_compile docling/datamodel/settings.py tests/test_settings_load.py
• python3 scripts/check_tach_module_coverage.py
• python3 scripts/check_max_lines.py --max-lines=1500
• Import smoke: python3 -c "import docling"
• Not run locally: the repository uv-managed pytest and Ruff checks; uv, pytest, and Ruff are unavailable in this environment.

Checklist

• Tests added for the behavior change.
• Documentation is not required for this internal default change.
• Commit includes the required DCO sign-off.

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