Ci test fix

[cs01]

ci: run unit tests in CI, smarter test.js setup checks

Summary

• compiler.test.ts and unit tests were never run in CI — only self-hosting and examples were. A broken unit test would pass CI.
• npm test unconditionally rebuilt the entire compiler every run, even when nothing had changed.

Changes

• ci.yml: add npm test step (10min timeout) to both build-linux-glibc and build-macos jobs, after native chad is built and before examples/self-hosting
• scripts/test.js: replace always-rebuild with staleness checks — only rebuilds dist/ if src/**/*.ts is newer, only builds .build/chad if missing; fails fast with a clear error + fix command if vendor artifacts are missing

Test plan

• CI passes on Linux and macOS
• npm test skips rebuild when dist/ is fresh
• npm test fails immediately with a clear message if vendor artifacts are missing