Add per-instance visual texture randomization

[yuecideng]

Description

This PR adds per-instance base-color texture randomization for vectorized environments (num_envs > 1). It supports deterministic fixed assignments, random selection, cycling, and sampling without replacement, with targeted environment selection and texture-reference caching.

It also documents the new randomization parameters and adds focused integration and cache tests.

Dependencies: None

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which improves an existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (existing functionality will not work without user modification)
• Documentation update

Screenshots

Not applicable.

Checklist

• I have run the black . command to format the code base.
• I have made corresponding changes to the documentation.
• I have added tests that prove my feature works.
• Dependencies have been updated, if applicable.

Verification: focused visual-randomization tests pass (9 passed); the full gym/object suite passed with 339 passed and 69 skipped.