perf: only copy jsx-ast's input on the main thread

Copying every chunk's input with one thread cost ~2.5s of a ~24s build,
while only `jsx-ast` needs it: building a page changes its entries, and
on the main thread those are the metadata other generators read. It now
copies its own input there, and workers keep using theirs as is.

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

Author: ovflowd

packages/core/src/threading/parallel.mjs

@@ -96,17 +96,10 @@ export default function createParallelWorker(
         { generator: name, chunks: chunks.length, chunkSize, threads }
       );
 
-      // On this thread, one chunk at a time, so their work isn't all held at
-      // once, and on a copy, as a worker gets, which the chunk can change
+      // On this thread, one chunk at a time, so their work isn't all held at once
       if (threads <= 1 || items.length <= 2) {
         for (const indices of chunks) {
-          const input = structuredClone(indices.map(i => items[i]));
-
-          yield await generator.processChunk(
-            input,
-            input.keys().toArray(),
-            extra
-          );
+          yield await generator.processChunk(items, indices, extra);
         }
 
         return;

packages/react/src/jsx-ast/generate.mjs

@@ -1,3 +1,5 @@
+import { isMainThread } from 'node:worker_threads';
+
 import getConfig from '@doc-kit/core/utils/configuration/index.mjs';
 import { groupNodesByModule } from '@doc-kit/core/utils/generators.mjs';
 import { jsx, toJs } from 'estree-util-to-js';
@@ -24,7 +26,11 @@ export async function processChunk(slicedInput, itemIndices) {
   const results = [];
 
   for (const idx of itemIndices) {
-    const { head, entries } = slicedInput[idx];
+    // Building a page changes its entries. A worker has its own copy, but on
+    // the main thread they're the metadata other generators read too
+    const { head, entries } = isMainThread
+      ? structuredClone(slicedInput[idx])
+      : slicedInput[idx];
 
     const { content, ...page } = await buildContent(entries, head);