[gouyt13]

Summary

Reduce CI setup and compilation overhead while retaining existing jobs, matrices, gates, and test commands.

• Install only clang-format 23 in the formatting job; keep the full LLVM/OpenMP tool setup as the default for other callers.
• Use preinstalled CMake/Ninja in six workflow steps instead of refreshing apt and reinstalling them, with explicit tool version checks.
• Enable /MP2 for the Windows C++ and core/FastScan consumer jobs, complementing the existing MSBuild project parallelism.
• Add regression coverage for LLVM package selection and installation/tool failure propagation.

Testing

• CI automation regression suite: 42 tests passed.
• Fresh CMake/Ninja source-consumer build: succeeded; CTest 3/3 passed.
• scripts/check-python.sh, Ruff checks for .github/scripts, and ShellCheck for the setup action: passed.
• actionlint: passed using a local configuration declaring the existing windows-2025-vs2026 runner label, which the installed lint version does not recognize by default.
• Compared workflow definitions: job names, dependencies, matrices, conditions, and existing test commands are unchanged.
• git diff --check: passed.

Windows execution and hosted-runner timing remain to be verified by this PR's CI. No measured speedup is claimed.