fix(core): copy esm-shims.js to dist during tsdown bundling

packages/core/__tests__/build-artifacts.spec.ts

@@ -0,0 +1,19 @@
+import fs from 'node:fs';
+import path from 'node:path';
+import url from 'node:url';
+
+import { describe, expect, it } from 'vitest';
+
+const coreDir = path.resolve(path.dirname(url.fileURLToPath(import.meta.url)), '..');
+const distDir = path.join(coreDir, 'dist');
+
+describe('build artifacts', () => {
+  it('should include esm-shims.js in dist for tsdown shims support', () => {
+    const shimsPath = path.join(distDir, 'esm-shims.js');
+    expect(fs.existsSync(shimsPath), `${shimsPath} should exist`).toBe(true);
+
+    const content = fs.readFileSync(shimsPath, 'utf8');
+    expect(content).toContain('__dirname');
+    expect(content).toContain('__filename');
+  });
+});

packages/core/build.ts

@@ -428,6 +428,11 @@ async function bundleTsdown() {
       }),
     ],
   });
+
+  // Copy esm-shims.js to dist/ so tsdown's shims option can resolve it.
+  // tsdown resolves this file via path.resolve(import.meta.dirname, '..', 'esm-shims.js'),
+  // which means it expects the file at dist/esm-shims.js (one level up from dist/tsdown/).
+  await copyFile(join(tsdownSourceDir, 'esm-shims.js'), join(projectDir, 'dist/esm-shims.js'));
 }
 
 async function brandTsdown() {