Add branding: Focus mark, wordmark, banner, social preview

- assets/branding/: icon (SVG + 512px PNG for Gitea avatar), adaptive-icon
  foreground/background layers, path-only wordmark for dark/light grounds,
  1200x300 README banner, 1280x640 social preview (SVG + PNG)
- README: banner hero
- build-status: branding decision log (includes the pending website section
  from the parallel web/ session, interleaved in the same file)

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

assets/branding/icon.svg

@@ -0,0 +1,24 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 96 96">
+  <defs>
+    <linearGradient id="tile" x1="0" y1="0" x2="0" y2="1">
+      <stop offset="0" stop-color="#0E2433"/>
+      <stop offset="1" stop-color="#071522"/>
+    </linearGradient>
+  </defs>
+  <rect width="96" height="96" rx="21" fill="url(#tile)"/>
+  <!-- the network, at rest -->
+  <g fill="#1E4A5C">
+    <circle cx="24" cy="24" r="2.6"/><circle cx="48" cy="24" r="2.6"/><circle cx="72" cy="24" r="2.6"/>
+    <circle cx="24" cy="48" r="2.6"/>                                <circle cx="72" cy="48" r="2.6"/>
+    <circle cx="24" cy="72" r="2.6"/><circle cx="48" cy="72" r="2.6"/><circle cx="72" cy="72" r="2.6"/>
+  </g>
+  <!-- one node, under examination -->
+  <circle cx="48" cy="48" r="8" fill="none" stroke="#FFB454" stroke-opacity="0.3" stroke-width="2"/>
+  <circle cx="48" cy="48" r="4.5" fill="#FFB454"/>
+  <g fill="none" stroke="#35E0C4" stroke-width="3.5" stroke-linecap="round" stroke-linejoin="round">
+    <path d="M 33 41 V 33 H 41"/>
+    <path d="M 55 33 H 63 V 41"/>
+    <path d="M 63 55 V 63 H 55"/>
+    <path d="M 41 63 H 33 V 55"/>
+  </g>
+</svg>