🌳 fix: keep the Symbol.metadata install through tree-shaking

`sideEffects` named metadata.js, the module holding the global install, but not the
barrel that re-exports it. A side-effect-free barrel is droppable whole, so every
bundler pruned the `export * from './metadata.js'` edge before metadata.js's own
marking was ever consulted, and the install vanished.

Measured on `import { configure } from 'cereale'`: 145 bytes through esbuild, 143
through webpack, 144 through rollup, with `Symbol.metadata` absent from all three.
It matters because tsc's decorator emit reads the well-known symbol directly —

    const _metadata = typeof Symbol === "function" && Symbol.metadata
      ? Object.create(null) : void 0;

so without it a decorated class is handed `metadata: undefined` and ends up with no
rules at all. Two places in the source promised this survived bundling; it did not.

index.js and index.ts are listed now. The cost is ~100 bytes and lands only on
imports that reach nothing else: the single-decorator, validateSync, toPlainSync,
toInstanceSync and whole-library cases are byte-identical before and after, across
all three bundlers. dist/cjs needs nothing — tsc emits `__exportStar(require(...))`,
an unconditional statement no bundler can drop.

Two new cases in treeshake.test.ts pin it, one on src and one on dist/esm, since
those are separate manifest paths and a typo in either is invisible from the other.
Both were checked by removing the entry and watching them fail.

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

package.json

@@ -23,8 +23,10 @@
     }
   },
   "sideEffects": [
+    "./dist/esm/index.js",
     "./dist/esm/metadata.js",
     "./dist/cereale.min.js",
+    "./src/index.ts",
     "./src/metadata.ts"
   ],
   "files": [