🔊 feat!: make the three silent failures loud

Every change here answers one question: where does cereale currently fail
without saying so?

**Vite 8 / Vitest 4 drop decorators silently.** Both transform with oxc,
which does not implement the standard decorator transform and does not
report that. `vitest` prints "0 test" beside a bare SyntaxError, and
`vite build` reports success while emitting a bundle that throws on first
import. Ship the plugin that fixes it as `cereale/vite`, transforming with
esbuild and falling back to tsc — cereale depends on neither. The library's
own suite now runs through it, so it is exercised by every test.

**Legacy decorators died opaquely.** With `experimentalDecorators: true`,
still the default in most existing TypeScript projects, decorators are
invoked as (prototype, "name") and cereale raised "TypeError: Cannot convert
undefined or null to object". All decorators now resolve metadata through
one checkpoint that names the tsconfig setting instead, and reject
application to a method, getter or accessor field.

**Values JSON cannot carry were emptied.** A populated Map serialized to
{}, a Uint8Array to index-keyed noise, a bigint straight through so the
caller's own JSON.stringify threw somewhere unrelated. All now raise
JsonMappingError naming the property path and both ways out. Covers what a
@JsonSerialize serializer returns, sync or async. Circular-reference and
depth errors name the path too.

Also fixed: defineRule on a subclass with no decorators of its own wrote
the rule into its base class, because the base's metadata object is
inherited through the static prototype chain and `??=` found it non-nullish.

The README's toolchain table (tsc, esbuild, swc ✅, oxc ❌) is now executed
by a test rather than asserted, and the positioning leads with
class-validator + class-transformer, the stack cereale actually replaces,
rather than Zod, which it deliberately is not.

193 -> 249 tests.

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

vitest.config.ts

@@ -1,34 +1,10 @@
 import { defineConfig } from 'vitest/config';
-import { transform } from 'esbuild';
+import { standardDecorators } from './src/vite.js';
 
 /**
- * Transpiles test sources with esbuild instead of oxc.
- *
- * Vitest 4 transforms with oxc, which does not yet implement the TC39 standard decorator
- * transform — it leaves the syntax in place and Node then fails to parse it, reporting
- * "0 test" rather than an error. esbuild and tsc both implement it, so the library's own
- * build (`tsc`) and consumers bundling with esbuild or Vite are unaffected; only the test
- * runner needs this. Remove it once oxc gains standard-decorator support.
+ * The library's own tests run through the plugin the library ships, so that `cereale/vite`
+ * is exercised by every test run rather than only by the one test that asserts it exists.
  */
-function standardDecorators() {
-  return {
-    name: 'cereale:standard-decorators',
-    enforce: 'pre' as const,
-    async transform(code: string, id: string) {
-      if (!/\.ts$/.test(id) || id.includes('node_modules')) return null;
-      const result = await transform(code, {
-        loader: 'ts',
-        target: 'es2022',
-        sourcefile: id,
-        sourcemap: true,
-        // Standard semantics, not the legacy ones: the library reads context.metadata.
-        tsconfigRaw: { compilerOptions: { experimentalDecorators: false, useDefineForClassFields: true } },
-      });
-      return { code: result.code, map: result.map };
-    },
-  };
-}
-
 export default defineConfig({
   plugins: [standardDecorators()],
   test: {