Test all examples with Valgrind in CI

[lukewilliamboswell]

Summary

• add a Valgrind mode to the spec-driven example runner
• fail on reported memory errors and leaks while preserving each case's expected exit code and output assertions
• run the existing x64musl matrix entry under Valgrind while keeping arm64 Linux, macOS, and Windows as normal executions
• suppress the known musl mallocng encoded-metadata check without suppressing callers

Closes #192

Testing

• ./scripts/test.py --operation build --target x64musl --artifact-dir /tmp/basic-cli-192-artifacts
• ./scripts/test.py --operation run --target x64musl --artifact-dir /tmp/basic-cli-192-artifacts --valgrind
• ./scripts/test.py --operation run --target x64musl --artifact-dir /tmp/basic-cli-192-artifacts
• parsed scripts/test.py and .github/workflows/ci.yml
• git diff --check