[FXC-4804] Support surface selection for custom force distribution

[yifan-flex]

Note

Introduces selective surface integration and configurable resolution for force distribution outputs.

• Adds optional entities (alias surfaces) and number_of_segments to ForceDistributionOutput and inherited TimeAverageForceDistributionOutput in outputs.py; validates surfaces exist
• Updates solver translation to emit surfaces and numberOfSegments in forceDistributionOutput and timeAveragedForceDistributionOutput
• Expands docs/examples for usage
• Adds/updates tests for validation and translation, including defaults (numberOfSegments: 300) and custom values

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