test: cover CMake subdirectory consumers

[Hrushikesh-ramilla]

Summary

Add the CMake consumer regression coverage requested in #3758. The target header interfaces are already present; this change protects them with a separate parent project that links libzstd_static and libzstd_shared without manually supplying include paths.

Both C++98 consumers include zstd.h, zstd_errors.h, and zdict.h, compress and decompress a buffer, and verify the round trip. The existing Linux, macOS, and Windows paths-with-spaces jobs configure, build, and run the consumers. On Windows, the shared consumer receives its DLL next to the executable.

A separate CI repair commit fixes two setup failures encountered during fork validation:

• Request platform-tools explicitly from the pinned Android setup action, avoiding the removed tools package. The existing NDK installation and builds remain in place.
• Select Visual Studio 2026 for the native Windows ARM64 runner, matching its installed image. The other Windows matrix entries retain their existing generators.

Relates to #3758 and #2377.

Validation

• MSVC 19.42 x64, Windows SDK 10.0.22621, CMake 3.30.6: both consumers build with warnings treated as errors and pass CTest.
• Temporarily removing the static and shared target include interfaces makes both consumers fail with a missing zstd.h; restoring the interfaces makes both pass again.
• MSVC /analyze passes for both consumer programs.
• License headers and git diff --check pass.
• Full GitHub Actions validation at c9e787b: all 104 check jobs pass across all five workflows (commit, short tests, long tests, CMake, and Android NDK).
• Fork validation: test: cover CMake subdirectory consumers Hrushikesh-ramilla/zstd#1

AI assistance: Codex assisted with the regression fixture, workflow changes, and validation.