perf(core): end idle workers after 500ms instead of a second

A worker only ended a second after its last task, so the `jsx-ast`
workers and their heaps (~800MB on the Node.js docs) were still there
while the main thread bundled the site, which is when the build peaked.
They now end half a second after running out of work: soon enough to be
gone while the site is bundled, but not in the short gaps between two
generators, where ending and starting workers again costs time.

On Node core's build with 4 threads on Node 26, the peak goes from
~2.31GB to ~1.91GB.

Assisted-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: ovflowd

packages/core/src/threading/index.mjs

@@ -24,7 +24,10 @@ export default function createWorkerPool(threads) {
     filename: workerScript,
     minThreads: 0,
     maxThreads: threads,
-    idleTimeout: 1_000,
+    // A worker idle for half a second ends, so its heap is gone during a long
+    // stretch of work on the main thread, such as bundling the site, while the
+    // short gaps between generators leave it running
+    idleTimeout: 500,
     resourceLimits: { maxOldGenerationSizeMb: WORKER_HEAP_SIZE },
     workerData: { logLevel: logger.getLogLevel() },
   });