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>

public/legacy/ads/magewell-300x250.svg

@@ -0,0 +1,28 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 300 250" role="img" aria-label="Magewell">
+  <defs>
+    <linearGradient id="mw-bg" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0" stop-color="#0a0a0a"/>
+      <stop offset="1" stop-color="#1a1a1a"/>
+    </linearGradient>
+    <linearGradient id="mw-accent" x1="0" y1="0" x2="1" y2="0">
+      <stop offset="0" stop-color="#dc2626"/>
+      <stop offset="1" stop-color="#991b1b"/>
+    </linearGradient>
+  </defs>
+  <rect width="300" height="250" fill="url(#mw-bg)"/>
+  <rect x="0" y="0" width="300" height="4" fill="url(#mw-accent)"/>
+  <rect x="0" y="246" width="300" height="4" fill="url(#mw-accent)"/>
+
+  <!-- wordmark -->
+  <text x="150" y="115" text-anchor="middle" font-family="Helvetica, Arial, sans-serif" font-size="40" font-weight="800" fill="#ffffff" letter-spacing="-1">Magewell</text>
+
+  <!-- tagline -->
+  <text x="150" y="148" text-anchor="middle" font-family="Helvetica, Arial, sans-serif" font-size="13" font-weight="500" fill="#dc2626" letter-spacing="2">PROFESSIONAL VIDEO I/O</text>
+
+  <!-- body -->
+  <text x="150" y="186" text-anchor="middle" font-family="Helvetica, Arial, sans-serif" font-size="12" fill="#9ca3af">Capture · Convert · Stream</text>
+
+  <!-- CTA chip -->
+  <rect x="100" y="200" width="100" height="28" rx="4" fill="#dc2626"/>
+  <text x="150" y="219" text-anchor="middle" font-family="Helvetica, Arial, sans-serif" font-size="12" font-weight="700" fill="#ffffff" letter-spacing="2">EXPLORE →</text>
+</svg>