✨ 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/example.ts

@@ -25,8 +25,7 @@ import {
   Validate,
   ValidatorConstraintInterface,
   ValidationArguments,
-  registerDecorator,
-  ValidationOptions
+  Matches,
 } from './index.js';
 
 // --- Custom Validators ---
@@ -42,17 +41,8 @@ class IsLongerThan implements ValidatorConstraintInterface {
   }
 }
 
-function IsSlug(options?: ValidationOptions) {
-  return function (object: any, propertyName: string) {
-    registerDecorator({
-      name: 'isSlug',
-      target: object.constructor,
-      propertyName: propertyName,
-      ...(options ? { options } : {}),
-      validator: (value: any) => typeof value === 'string' && /^[a-z0-9-]+$/.test(value)
-    });
-  };
-}
+/** A custom rule is just a decorator that composes an existing one. */
+const IsSlug = () => Matches(/^[a-z0-9-]+$/, { message: 'name must be a lowercase slug' });
 
 // --- Custom Serializers ---
 
@@ -76,12 +66,14 @@ enum Format {
 }
 
 abstract class Media {
+  // Standard decorators cannot be applied to an `abstract` member, so the discriminator is a
+  // concrete field the subclasses override.
   @IsString()
-  abstract type: string;
+  type: string = '';
 
   // Declared once here. Subclasses inherit the rule without restating it.
   @IsString()
-  title: string;
+  title: string = '';
 }
 
 class Book extends Media {
@@ -111,7 +103,7 @@ class Movie extends Media {
   duration: number;
 
   // Only checked for films that claim to be part of a series.
-  @ValidateIf((movie: Movie) => movie.duration > 200)
+  @ValidateIf<Movie>(movie => movie.duration > 200)
   @IsString()
   intermissionNote?: string;
 }
@@ -122,7 +114,7 @@ class Library {
   id: string;
 
   @IsString()
-  @IsSlug({ message: 'name must be a lowercase slug' })
+  @IsSlug()
   name: string;
 
   @JsonProperty('curator_email')
@@ -136,11 +128,12 @@ class Library {
 
   @IsArray()
   @ValidateNested({ each: true })
-  @JsonPolymorphic('type', [
+  // Naming the base type has the subtype list checked against it.
+  @JsonPolymorphic<Media>('type', [
     { value: Book, name: 'book' },
     { value: Movie, name: 'movie' }
   ])
-  items: Media[];
+  items: Media[] = [];
 }
 
 // --- Execution ---