feat: add optional physicsUsd Box declaration package

[snkmcb]

Summary

Add an optional installed physicsUsd::physicsUsd package that reads standard USD Box declarations into validated, backend-neutral snapshots. Stage Runner can use the package without owning reusable USD physics interpretation or adding a sibling source dependency.

The first slice supports Cube collision, static/dynamic bodies, explicit positive dynamic mass, and a bounded Y-up meter/kg transform contract. Unsupported declarations, geometry, instancing, units, and transforms produce typed diagnostics before any runtime resources are created. Core and Jolt remain independent of OpenUSD.

Add package exports, an OpenStrata member and usd build intent, boundary and installed-consumer checks, and update architecture/roadmap documentation. The root build remains core/backend-only by default; enable the adapter with USDPHYSICS_BUILD_USD=ON.

Validation

• Windows MSVC Release / OpenUSD 26.08: 12 CTest tests passed.
• Rechecked affected parser, boundaries, documentation, versions, and clean-prefix consumer after the final parser changes: 5/5 passed.
• Downstream Stage Runner: 49/49 tests passed with standard import enabled, including 180-frame compatibility comparisons and the usdview native adapter.
• ost ci validate and git diff --check passed.

Delivery scope

Linux/hosted bridge verification and publishing/pinning physicsUsd artifacts remain follow-up work. Scenes/gravity, mass inference, additional geometry, joints, composed transforms, retained mappings, and synchronization records are outside this bounded slice.

Companion Stage Runner integration: animu-sphere/usd-stage-runner#31