Bound plot3D isosurfaces for grayscale structures

[CAOShurong]

Summary

• add a public nlevels argument to Simulation.plot3D for both the native and HTML backends
• default to at most 10 evenly distributed non-background permittivity levels while preserving all levels when fewer are present
• reject non-positive or non-integral limits and document the bounded behavior
• add regression coverage for the default cap, explicit limits, background exclusion, validation, and HTML-backend forwarding

Testing

• base reproduction: a 12-level grayscale grid generated 11 non-background isosurfaces
• committed-source marching-cubes probe: default generated 10 surfaces; nlevels=3 generated levels 2, 7, and 12
• focused test_plot3D_nlevels execution and HTML dispatch probe
• pre-commit hooks for all four changed files
• python -m compileall for the changed Python sources and test
• python -m mkdocs build --strict -f mkdocs.yml
• git diff --check

Closes #3317

Disclosure: This contribution was developed with OpenAI Codex assistance. I reviewed the implementation and regression coverage.

[oskooi]

LGTM.