ads: restore Magewell 300x250 banner with placeholder SVG creative

Magewell was removed when Adsanity went away; PENDING_ads.md flagged it
along with AJA/Zixi/Lectrosonics/Opengear/Studio Hero for real creatives.
This restores Magewell with an in-house SVG placeholder (red/dark Magewell
brand color, wordmark + tagline + CTA chip) so the sidebar 300x250 stack
gets three rotating entries again instead of two. Real Magewell creative
to land when sourced from the vendor.

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

src/lib/ads.ts

@@ -30,9 +30,10 @@ export const ADS_728X90: Ad[] = [
  * stacks them vertically and shuffles the order per page-view so each banner
  * cycles through positions. Add new entries here to grow the stack.
  *
- * AJA, Zixi, Lectrosonics, Opengear, Studio Hero, Magewell were removed
- * pending real creatives (their Adsanity campaigns relied on
- * ads.broadcastbeat.com, which is down). See PENDING_ads.md.
+ * AJA, Zixi, Lectrosonics, Opengear, Studio Hero were removed pending real
+ * creatives (their Adsanity campaigns relied on ads.broadcastbeat.com,
+ * which is down). Magewell now has an SVG placeholder pending a real
+ * creative. See PENDING_ads.md.
  */
 export const ADS_300X250: Ad[] = [
   {
@@ -42,6 +43,13 @@ export const ADS_300X250: Ad[] = [
     alt: "Telycam Mix One ExploreXE NAB 2026",
     click_url: "https://telycam.com/",
   },
+  {
+    label: "Magewell",
+    size: "300x250",
+    src: "/legacy/ads/magewell-300x250.svg",
+    alt: "Magewell — professional video I/O",
+    click_url: "https://www.magewell.com/",
+  },
   {
     label: "LiveU",
     size: "300x250",