[piggy-penguin]

Summary

• Adds a test-only gstest: URI resolver plugin (plugins/gaussian-sog/tests/uri_resolver/GsTestUriResolver.cpp). CTest builds it into its own build directory next to a generated plugInfo.json and registers it through PXR_PLUGINPATH_NAME for one new suite only. It is never installed or packaged.
• The resolver maps gstest:<path> onto a store directory. It serves every asset as a stream only (GetBuffer() returns null) and logs each OpenAsset/GetBuffer call.
• Adds a new suite, gaussianSog_python_uri_resolver (test_gaussian_sog_uri_resolver.py), which checks:
  • opening a meta.json and a .sog by URI, directly, by reference from a filesystem layer, and metadata-only;
  • every companion is resolved and served by the URI resolver and read through ArAsset::Read();
  • a plane missing from the store gives GSSOG-E008, even when valid planes with the same name exist elsewhere;
  • a filesystem meta.json never reaches the URI resolver.
• Docs: the three ⬜ rows of RESOLVER_MATRIX.md become ✅, plus two new rows. Also updates current.md, BUILDING.md (16 suites) and CHANGELOG.md.

Why?

docs/roadmap/current.md lists the remaining work on the resolver test matrix: "a test resolver plugin for URI-style identifiers and resolver-served companions". Three rows were covered only by the injected-loader seams, never by a real resolver plugin: a URI-style primary identifier, companions served by a custom ArResolver, and a stream without GetBuffer().

Validation

• Local run on Linux: the composed root tree built against the usd-core 26.8 wheel's libusd_ms and the OpenUSD v26.08 headers. All five gaussianSog_* CTest suites pass. (gaussianUsd_tests does not link in that setup because the wheel expects libpython to be loaded already. That is a local artifact, unrelated to this change.)
• Mutation checks:
  • removing the ArAsset::Read() fallback for companions fails only the new suite;
  • reading the primary asset from the filesystem fails both resolver suites.
• python scripts/check_clang_format.py is clean.
• Hosted: the workspace-ctest-pr-* cells in this PR are the Windows/macOS/Linux check.

Checklist

• I linked related context or explained why there is no issue.
• I ran the relevant tests and checks, or described what remains.
• I updated documentation or tests when behavior changed.
• I did not add private or unlicensed assets.

Reviewer notes

No plugin code changes. Only tests, a test-only plugin, and docs. Windows (MSVC, DLL in a multi-config output dir) has not been built locally, so that first build happens in the hosted cell.

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