perf: use up to 6 threads by default

With generators streaming into each other, more threads now pay off.
On Node.js's docs, medians of 5 runs, the site build took 7.64s at 4
threads, 6.48s at 6 and 6.66s at 8, peaking at 1.9GB, 2.3GB and 2.7GB.
The legacy build kept getting faster at 8, but by only another 5% for
320MB more. Machines with fewer CPUs are unaffected, as the default is
the CPU count up to this ceiling.

Keeping Rolldown and Rayon at 4 threads while doc-kit used 6 or 8 was
slower and saved no memory, so the bundler still gets doc-kit's count.

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

Author: ovflowd

.changeset/six-threads.md

@@ -0,0 +1,5 @@
+---
+'@doc-kit/core': minor
+---
+
+Builds use up to 6 threads by default, instead of 4: with generators streaming into each other, more threads now pay off. On the Node.js API docs, the site build is ~15% faster at 6 threads than at 4, for ~20% more memory, while 8 threads were no faster than 6. `--threads` still sets any count.

docs/configuration.md

@@ -134,7 +134,7 @@ these for that generator alone, except `markdown`, which it adds to.
 Top-level, alongside `target` and `global`:
 
 - `threads` {number} Worker threads used for generation. Defaults to your
-  CPU count. Each worker's heap is limited to 512MB; to raise the limit, run
+  CPU count, up to 6. Each worker's heap is limited to 512MB; to raise the limit, run
   Node.js with `--max-old-space-size` (directly or through `NODE_OPTIONS`).
 - `chunkSize` {number} Items processed per worker thread. **Default:** `10`.
 

packages/core/src/threading/constants.mjs

@@ -3,9 +3,10 @@
 /**
  * The default `threads` ceiling. Each worker holds a heap of its own, with the
  * libraries and the pages it is working on, so past a few threads memory, not
- * CPU, is what runs out. `--threads` raises it.
+ * CPU, is what runs out: on the Node.js docs, 8 threads were no faster than 6,
+ * for ~15% more memory. `--threads` raises it.
  */
-export const DEFAULT_MAX_THREADS = 4;
+export const DEFAULT_MAX_THREADS = 6;
 
 /**
  * The heap size limit of each worker thread (V8's old space), in MB. V8 lets a