Vendored OpenUSD proposals: UsdSolid and cad_geometry

UsdSolid/ and cad_geometry/ are unmodified snapshots of two proposals from the
Alliance for OpenUSD (AOUSD) Geometry Working Group, submitted to Pixar's
OpenUSD-proposals repository on 2026-05-20:

- cad_geometry: "Boundary Representation Geometry in OpenUSD - Problem
  Statement", pull request #108
  https://github.com/PixarAnimationStudios/OpenUSD-proposals/pull/108
  Copyright (c) 2026, Alliance for OpenUSD (AOUSD), Geometry Working Group.

- UsdSolid: "Solid Models in USD", pull request #109
  https://github.com/PixarAnimationStudios/OpenUSD-proposals/pull/109

The snapshot is commit 811f03c603b06d3b175515a4bf991dee8f42f651, the head of
pull request #109, which also carries cad_geometry unchanged from pull
request #108.

Proposals submitted to OpenUSD-proposals are subject to the OpenUSD Technical
Proposal Supplemental Terms:
https://openusd.org/release/contributing_supplemental.html
