restore AVP packages and complete the repository index

[zc-xzc]

Summary

• restore the complete 3d_print_parts/avp_model package: BOM, illustrated guides, images, HTML model, and STP assembly
• restore the updf_Robotic/avp_model ROS/URDF/MuJoCo reference package
• restore the latest AVP meshes and model integration in G1_O6_combined_001
• restore the MIT LICENSE that was previously merged
• add every restored package to the root README and clarify its role
• clarify again that legacy filenames containing 3dof still belong to a physical 2-DOF yaw/pitch mechanism

Root cause

Commit 796e5d4, merged through PR #2, was based on an incomplete tree. Alongside the intended filename cleanup it removed the AVP print package, AVP simulation package, MIT license, and newer G1 AVP mesh/model changes that had already existed on main.

Important model distinction

The physical avp_model assembly is 2-DOF. Its standalone URDF currently uses fixed joints and its standalone MJCF is a static viewer model, so the README now distinguishes physical mechanism DOF from the exported simulation joint definitions.

Validation

• compared the current main tree against the last complete main commit before the accidental removal
• restored 46 asset/model/license entries byte-for-byte from their original Git blobs
• updated two README files intentionally
• confirmed the branch is one commit ahead of current main and changes 48 files total: 45 added, 3 modified
• ran Markdown whitespace checks and verified the restored object hashes