fix(legacy-html): copy its input on the main thread

Building a page splices the heading out of its entries' content and
replaces the stability nodes, and on the main thread those entries are
the metadata other generators read. With one thread, building
`legacy-html` with other targets crashed (`Cannot read properties of
undefined (reading 'tagName')`), on `main` too. It now works on a copy
there, as it already does in a worker.

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

Author: ovflowd

.changeset/keep-memory-down.md

@@ -1,6 +1,7 @@
 ---
 '@doc-kit/core': patch
 '@doc-kit/generator-react': patch
+'@node-core/doc-kit-legacy': patch
 ---
 
-Builds use less memory, most of all with few threads: each worker's young generation is limited to 96MB, a single thread processes one chunk at a time and lets go of the pages' code before rendering them, and Vite's native bundler gets as many threads as doc-kit and returns the memory it frees. On the Node.js API docs, `--threads 1` now builds with Node.js's `--max-old-space-size=512`.
+Builds use less memory, most of all with few threads: each worker's young generation is limited to 96MB, a single thread processes one chunk at a time and lets go of the pages' code before rendering them, and Vite's native bundler gets as many threads as doc-kit and returns the memory it frees. On the Node.js API docs, `--threads 1` now builds with Node.js's `--max-old-space-size=512`, and no longer crashes when `legacy-html` is built alongside other targets.

packages/node-legacy/src/legacy-html/generate.mjs

@@ -2,6 +2,7 @@
 
 import { readFile, cp } from 'node:fs/promises';
 import { basename, join } from 'node:path';
+import { isMainThread } from 'node:worker_threads';
 
 import getConfig from '@doc-kit/core/utils/configuration/index.mjs';
 import { writeFile } from '@doc-kit/core/utils/file.mjs';
@@ -33,7 +34,11 @@ export async function processChunk(slicedInput, itemIndices, navigation) {
   const results = [];
 
   for (const idx of itemIndices) {
-    const { head, nodes, headNodes } = slicedInput[idx];
+    // Building the content changes the entries. A worker has its own copy, but
+    // on the main thread they're the metadata other generators read too
+    const { head, nodes, headNodes } = isMainThread
+      ? structuredClone(slicedInput[idx])
+      : slicedInput[idx];
 
     const nav = navigation.replace(
       `class="nav-${head.api}"`,