🔧 fix: repair test, build and CI infrastructure

The vitest suite silently ran zero tests: vitest 4 transpiles with oxc, which
does not pick up `experimentalDecorators` from a tsconfig that excludes the
files being transformed, so every decorator-using suite failed to parse and was
reported as "0 test". A vitest.config.ts enabling legacy decorators brings all
40 existing tests back to life.

- Add vitest.config.ts (oxc legacy decorators + v8 coverage config)
- Type-check test files: move the test/demo exclusions from the base tsconfig
  onto the two build configs, and fix the strict-mode errors this surfaced
- Stop shipping src/example.ts in dist (it invokes runExample() at import time,
  a side effect in a package declaring "sideEffects": false)
- CI: run lint, tests with coverage, build and entry-point smoke checks; drop
  EOL Node 18, add Node 24; commit package-lock.json so `npm ci` works
- Add `verify`, `test:watch` and `build:docs` scripts, and an engines field;
  `build:docs` regenerates the previously hand-maintained docs/cereale.js

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SAcqrz3FcadkYr3xG32CjK

tsconfig.cjs.json

@@ -5,5 +5,6 @@
     "moduleResolution": "Bundler",
     "outDir": "dist/cjs",
     "declaration": true
-  }
+  },
+  "exclude": ["node_modules", "dist", "src/**/*.test.ts", "src/example.ts"]
 }