fix: add cmake subproject integration check

[feihongxu0824]

Summary

• Add a linux-x64 CMake subproject smoke check that configures zvec through an outer add_subdirectory() project.
• Replace zvec-owned CMAKE_SOURCE_DIR path assumptions in DiskANN and FastPFOR with PROJECT_ROOT_DIR so the smoke check can pass from a standalone branch off main.

Note: the DiskANN path changes overlap with #464 because this branch is intentionally based directly on main; the new subproject check would fail on current main without those fixes.

Testing

• ruby -e "require 'yaml'; YAML.load_file('.github/workflows/03-macos-linux-build.yml'); puts 'yaml ok'"
• Attempted local CMake subproject configure on macOS arm64; it reached the external subproject path and then failed because sandboxed network prevented FastPFOR's ARM-only SIMDe FetchContent download. The CI check runs on linux-x64 and does not take that ARM download path.
• Pre-commit hooks passed during commit.