Add USD input block with packaged importer

[alexchuber]

Summary

• Add UsdInputBlock for USD, USDA, USDC, and USDZ using the existing Babylon scene-loader → GLB → Document roundtrip.
• Update Babylon dependencies to 9.27.1 and package the USD runtime through pinned, checksum-verified build-time downloads, with cached assets and license notices. No runtime CDN dependency.
• Document the block and its Web Worker requirement in the block list. No worker adapter or CLI changes.

Validation

• Existing input/codec integration tests, downstream browser consumer tests, build, lint, and typecheck passed during implementation.
• A real browser USD pipeline produced one mesh with three positions, with external HTTPS blocked.
• Reviewed by Sol High agents; removed extra test infrastructure and added bounded build-time downloads.