perf(browser): Parallelize CDN bundle builds

Add a lightweight parallel rollup runner that processes config arrays
concurrently using the rollup JS API. Use it for @sentry/browser's
build:bundle which runs 93 rollup builds (31 entrypoints × 3 variants).

This reduces the browser bundle build from ~175s to ~65s (~2.6x faster),
which was the critical path for the full `yarn build`.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mydea

dev-packages/rollup-utils/rollupParallel.mjs

@@ -0,0 +1,34 @@
+/**
+ * Runs rollup builds in parallel for configs that export an array.
+ * Usage: node rollupParallel.mjs <config-path>
+ */
+import { cpus } from 'os';
+import { pathToFileURL } from 'url';
+import { resolve } from 'path';
+import { rollup } from 'rollup';
+
+const configPath = process.argv[2];
+if (!configPath) {
+  console.error('Usage: node rollupParallel.mjs <config-path>');
+  process.exit(1);
+}
+
+const { default: configs } = await import(pathToFileURL(resolve(configPath)).href);
+const concurrency = cpus().length;
+const queue = [...configs];
+let done = 0;
+
+async function worker() {
+  while (queue.length > 0) {
+    const config = queue.shift();
+    const bundle = await rollup({ ...config, onwarn: () => {} });
+    await bundle.write(config.output);
+    await bundle.close();
+    done++;
+    process.stdout.write(`\r  [${done}/${configs.length}] builds completed`);
+  }
+}
+
+console.log(`Running ${configs.length} rollup builds (concurrency: ${concurrency})...`);
+await Promise.all(Array.from({ length: concurrency }, worker));
+console.log('\nDone.');

packages/browser/package.json

@@ -57,7 +57,7 @@
   "scripts": {
     "build": "run-p build:transpile build:bundle build:types",
     "build:dev": "run-p build:transpile build:types",
-    "build:bundle": "rollup -c rollup.bundle.config.mjs",
+    "build:bundle": "node ../../dev-packages/rollup-utils/rollupParallel.mjs rollup.bundle.config.mjs",
     "build:transpile": "rollup -c rollup.npm.config.mjs",
     "build:types": "run-s build:types:core build:types:downlevel",
     "build:types:core": "tsc -p tsconfig.types.json",