This improves CI pipeline with toolchain caching, parallel builds, and error handling. It separates lint job to avoid duplication across matrix builds, adding concurrency control, job timeouts, and least-privilege permissions.

Summary by cubic

Refined CI to run faster and fail safer with separate linting, cached toolchains, and parallel matrix builds. Adds cancel-in-progress, strict timeouts, and least-privilege permissions, plus more robust test summarization.

• Refactors
  • Split out a fast lint job (clang-format/shfmt only) that gates matrix tests.
  • Cache the riscv toolchain per toolchain type; run GNU with full tests and LLVM as build-only.
  • Add workflow concurrency to cancel duplicate runs and set job timeouts.
  • Use least-privilege permissions and actions/checkout@v5.
  • Speed up builds with make -j and stricter error handling (set -euo pipefail).
  • Harden test summary: guard upload/comment if generated, clearer final status, and standardize semaphore results to "Overall: PASS" with criteria line (semaphore:overall=skipped).

^{Written for commit 26465ce. Summary will update automatically on new commits.}