fix(docs-layout): untrack headings write too, same self-cycle risk

The headings $effect had the same shape as the breadcrumb effect: it
writes `headings = extractHeadings(contentElement)` from inside an
`$effect`, and Svelte 5's `$state` assignment reads the current value
to dedupe equal writes. That hidden read would subscribe the effect
to its own write and re-fire on every doc page.

Same `untrack` fix. Compute `next` outside, write inside `untrack`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jaysin586

src/lib/components/DocsLayoutV2.svelte

@@ -183,7 +183,14 @@
     let headings: TocHeading[] = $state([])
 
     const refreshHeadings = () => {
-        if (contentElement) headings = extractHeadings(contentElement)
+        if (!contentElement) return
+        const next = extractHeadings(contentElement)
+        // Untracked to avoid the same self-cycle the breadcrumb effect can hit:
+        // assignment to a `$state` reads the existing value to dedupe, and that
+        // read inside an `$effect` would subscribe the effect to its own write.
+        untrack(() => {
+            headings = next
+        })
     }
 
     $effect(() => {