remove CAD HTML previews with unverified licensing

[zc-xzc]

Summary

• remove both self-contained CAD-generated HTML model previews
• retain all STEP/STP, STL, BOM, image, software, and simulation assets
• replace HTML references in the root README and four assembly documents with STEP/STP and STL guidance
• preserve the latest printable-part exports added in PR update active vision platform model exports #8
• restore the explicit clarification that the physical AVP is 2-DOF and that 3dof is only a legacy filename
• remove one already-broken assembly-image reference while updating the compact guide

Why

The HTML files embed the ZwViewer runtime and carry a ZWSOFT copyright notice. The user disclosed that the exports were generated with an unauthorized copy of the CAD software, and no valid license or redistribution authorization is available for the embedded viewer code. Removing the HTML exports is the conservative compliance action.

Scope

This PR removes the files from the current main tree. It does not rewrite Git history or remove copies from historical commits and old branches.

Validation

• confirmed the branch contains zero HTML files
• confirmed all STEP/STP assembly and printable-part files remain
• confirmed no Markdown file in scope still references either removed HTML file
• verified the branch is one commit ahead of current main and changes exactly seven files