🔧 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

vitest.config.ts

@@ -0,0 +1,20 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  // Vitest 4 transpiles with oxc, which does not read `experimentalDecorators`
+  // out of tsconfig.json for files the tsconfig does not `include`. Without this
+  // the decorator syntax in the test files fails to parse and every suite is
+  // silently reported as "0 test".
+  oxc: {
+    decorator: { legacy: true },
+  },
+  test: {
+    include: ['src/**/*.test.ts'],
+    coverage: {
+      provider: 'v8',
+      reporter: ['text', 'lcov'],
+      include: ['src/**/*.ts'],
+      exclude: ['src/**/*.test.ts', 'src/example.ts', 'src/index.ts'],
+    },
+  },
+});