✨ feat!: v2 — strongly typed decorators on the TC39 standard

BREAKING CHANGE: cereale moves from legacy `experimentalDecorators` to TC39
standard decorators, which is what makes validation rules type-checked against
the fields they are attached to.

    class User {
      @IsString() name!: string;   // fine
      @IsString() age!: number;    // Type 'number' is not assignable to 'string'
    }

Legacy decorators receive (target: any, key: string) and lose the field type
entirely, so this was impossible in v1. Standard decorators receive
ClassFieldDecoratorContext<This, Value>, which carries it. Rules now checked:
scalar rules against scalar fields; { each: true } against arrays, in both
directions; @JsonType against the field's class; @JsonSerialize/@JsonDeserialize
against the field's type; @IsIn and @IsEnum against the field's value type.

17 tests invoke the real compiler to assert the wrong code stays rejected — a
guarantee nobody checks is one that quietly stops holding.

Positioning follows the capability: validated domain objects, not validated
data. The README now leads with the Zod comparison. Cereale does not infer your
type from a schema — you still write the field type and the rule — but it
guarantees the two cannot disagree, which is what class-validator never offered.

Removed
- metadata-storage.ts and its WeakMap singleton. Metadata lives on
  context.metadata now, which also removes the dual ESM/CJS double-singleton
  hazard. Inheritance merging becomes structural rather than reconstructed on
  every read, so the subclass-shadowing defect fixed by hand in 0.1.0 cannot
  reoccur by construction.
- registerDecorator, replaced by defineRule(Class, 'field', constraint).

Unchanged: the engine, options, naming strategies, access control, error
helpers, the sync API, and the performance work. 193 tests pass.

Toolchain note: standard decorators are transformed by tsc and esbuild, but not
yet by oxc. The library builds with tsc and consumers on esbuild/Vite are fine;
Vitest 4 uses oxc, so the test runner needs an esbuild transform plugin. This is
recorded in vitest.config.ts and the README, and is the reason 1.x should stay
available for oxc-based toolchains.

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

src/index.test.ts

@@ -41,16 +41,18 @@ class DateDeserializer implements JsonDeserializer<string, Date> {
 
 // --- Domain Models ---
 abstract class Media {
+  // Standard decorators cannot be applied to an `abstract` member, so the base declares a
+  // concrete field the subclasses override.
   @IsString()
-  abstract type: string;
+  type: string = '';
 
   @IsString()
-  title: string;
+  title: string = '';
 }
 
 class Book extends Media {
   @IsString()
-  type: string = 'book';
+  override type: string = 'book';
 
   @IsString()
   author: string;
@@ -63,7 +65,7 @@ class Book extends Media {
 
 class Movie extends Media {
   @IsString()
-  type: string = 'movie';
+  override type: string = 'movie';
 
   @IsInt()
   @Min(1)
@@ -162,7 +164,7 @@ describe('JsonMapper', () => {
 
       @ArrayNotEmpty()
       @IsIn(['admin', 'user', 'guest'], { each: true })
-      roles: string[];
+      roles!: ('admin' | 'user' | 'guest')[];
 
       @IsUrl()
       @IsOptional()
@@ -224,7 +226,7 @@ describe('JsonMapper', () => {
       user.username = 'johndoe';
       user.email = 'john@example.com';
       user.active = true;
-      user.roles = ['superadmin'];
+      user.roles = ['superadmin' as 'admin'];
 
       const errors = await JsonMapper.validate(user);
       expect(errors).toHaveLength(1);