🔧 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

src/decorators.test.ts

@@ -238,7 +238,7 @@ describe('Additional Decorators', () => {
       t.val = 'wrong';
       const errors = await JsonMapper.validate(t);
       expect(errors).toHaveLength(1);
-      expect(errors[0].constraints['CustomValidator']).toBe('val must be correct');
+      expect(errors[0]!.constraints['CustomValidator']).toBe('val must be correct');
     });
   });
 
@@ -304,7 +304,7 @@ describe('Additional Decorators', () => {
       t.val = 5;
       const errors = await JsonMapper.validate(t);
       expect(errors).toHaveLength(1);
-      expect(errors[0].constraints['custom']).toBe('must be ten');
+      expect(errors[0]!.constraints['custom']).toBe('must be ten');
     });
 
     it('should handle options as second argument', async () => {
@@ -318,7 +318,7 @@ describe('Additional Decorators', () => {
       t.val = 2;
       const errors = await JsonMapper.validate(t);
       expect(errors).toHaveLength(1);
-      expect(errors[0].constraints['custom']).toBe('must be one');
+      expect(errors[0]!.constraints['custom']).toBe('must be one');
     });
   });
 
@@ -339,10 +339,10 @@ describe('Additional Decorators', () => {
         name: string;
       }
       const json = '[{"name": "a"}, {"name": "b"}]';
-      const items = await JsonMapper.fromJson(Item, json);
+      const items = (await JsonMapper.fromJson(Item, json)) as unknown as Item[];
       expect(Array.isArray(items)).toBe(true);
       expect(items[0]).toBeInstanceOf(Item);
-      expect(items[0].name).toBe('a');
+      expect(items[0]!.name).toBe('a');
     });
 
     it('should handle single polymorphic object', async () => {
@@ -350,7 +350,7 @@ describe('Additional Decorators', () => {
         @IsString() type: string;
       }
       class Dog extends Animal {
-        type = 'dog';
+        override type = 'dog';
         @IsString() breed: string;
       }
       class Test {
@@ -376,7 +376,7 @@ describe('Additional Decorators', () => {
       t.tags = ['a', 1 as any];
       const errors = await JsonMapper.validate(t);
       expect(errors).toHaveLength(1);
-      expect(errors[0].constraints['isString']).toContain('each element');
+      expect(errors[0]!.constraints['isString']).toContain('each element');
     });
   });
 });