ci: testshade --buildinfo, and show it in the GHA summary

[lgritz]

Add read-only ShadingSystem attributes describing how OSL was built, so any app linking liboslexec can retrieve them. These are modeled on OIIO's attributes of the same names:

• build:platform -- e.g. "MacOS/ARM"
• build:compiler -- e.g. "gcc 11.2"
• build:simd -- synonym for osl:simd
• build:dependencies -- semicolon-separated "name version" for every dependency the build looked for (from CMake, via a configured private header, buildopts.h)
• build:batched -- batched SIMD targets compiled in
• hw:batched -- the subset of those this hardware can run

Add testshade --buildinfo, which prints those attributes plus the running hardware (and GPU, for OptiX builds). It mirrors oiiotool --buildinfo. For example:

OSL 1.16.0.0dev | Linux/x86_64
    Build compiler: gcc 11.2 | C++17/201703
    HW features enabled at build: sse2,sse3,ssse3,sse41,sse42,avx,avx2,f16c
    Back ends: x86_64 single, batched b8_AVX2
    Dependencies: ZLIB 1.2.11, Imath 3.1.9, OpenImageIO 3.0.11.0, pugixml 1.16, LLVM 15.0.7, partio, Python3 3.10.11, pybind11 2.9.0, Qt6 NONE, Qt5 5.15.9, Robinmap, BISON 3.0.4, FLEX 2.6.1
    Running on: 4 cores, 15.6 GB, SIMD sse2,sse3,ssse3,sse41,sse42,avx,avx2,avx512f,avx512dq,avx512ifma,avx512cd,avx512bw,avx512vl,fma,f16c,popcnt,rdrand

ci-test.bash appends this to $GITHUB_STEP_SUMMARY, as OIIO does (OIIO PR 5453), so each CI job's toolchain and dependencies are visible at a glance on the workflow summary page.

Also fix bugs in ShadingSystem::getattribute:

• "hw:simd" queried the nonexistent OIIO attribute "hw:string".
• "osl:simd" never reported neon.
• "osl:cuda_version" and "osl:optix_version" returned a raw char* rather than a ustring's chars, so retrieving them as a ustring gave garbage.

Assisted-by: Claude Code / Claude Opus 5.5