Normalize Unity heightfield data before writing to MuJoCo

[sylvesterkaczmarek]

Fixes #1930.

Summary

Unity heightfields with a maximum sample below 1 are currently scaled twice: the generated MuJoCo hfield size already uses the observed physical height range, while UpdateHeightFieldData() writes the original unnormalized samples into hfield_data.

This change:

• records the minimum and maximum source heightmap values during preparation;
• records MinimumHeight as well as MaximumHeight;
• normalizes programmatic hfield_data samples to [0, 1] using that source range;
• handles constant-height maps by writing zero normalized samples.

This keeps the MuJoCo heightfield data consistent with the physical range encoded in the generated hfield size.

Validation

• Final diff: 1 file, +17/-5 lines, one commit, with no unrelated changes.
• Branch is based on current upstream main at f7da3d295b828478e0171967a7fa3fe6bb2ec922.
• No dedicated Unity heightfield regression test exists in the repository. Unity tests were not run locally in this environment, so upstream CI and maintainer validation are required.