🔧 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

package.json

@@ -19,13 +19,19 @@
   ],
   "scripts": {
     "build": "rm -rf dist && tsc -p tsconfig.cjs.json && tsc -p tsconfig.esm.json && echo '{\"type\": \"commonjs\"}' > dist/cjs/package.json",
+    "build:docs": "esbuild src/index.ts --bundle --format=iife --global-name=Cereale --minify --tsconfig=tsconfig.json --outfile=docs/cereale.js",
     "demo": "node --no-warnings=ExperimentalWarning --loader ts-node/esm src/example.ts",
     "type-check": "tsc --noEmit",
     "test": "vitest run",
+    "test:watch": "vitest",
     "test:coverage": "vitest run --coverage",
     "lint": "eslint .",
     "lint:fix": "eslint . --fix",
-    "prepublishOnly": "npm run build"
+    "verify": "npm run type-check && npm run lint && npm run test && npm run build",
+    "prepublishOnly": "npm run verify"
+  },
+  "engines": {
+    "node": ">=20.0.0"
   },
   "repository": {
     "type": "git",
@@ -49,6 +55,7 @@
     "@eslint/js": "^10.0.1",
     "@types/node": "^25.6.0",
     "@vitest/coverage-v8": "^4.1.4",
+    "esbuild": "^0.25.0",
     "eslint": "^10.2.1",
     "globals": "^17.5.0",
     "ts-node": "^10.9.2",