Add setting to disable model previews

[ViTeXFTW]

Summary

• Add the zerosyntax.preview.enable setting to control rendered W3D model previews.
• Disable preview documentation and cached image generation when the setting is false.
• Apply the setting immediately through runtime configuration and document it in the VS Code extension.

Tests

• Updated Rust runtime-settings tests.
• Extended the end-to-end test to verify previews can be disabled without restarting.

Checklist

• I kept the change focused.
• I ran the relevant checks or explained why they were skipped.
• I did not commit generated artifacts, game source, corpus data, or secrets.

[greptile-apps]

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