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-adaptive-foreground.svg

@@ -0,0 +1,18 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 108 108">
+  <!-- Android adaptive-icon foreground layer: 108dp canvas, art kept inside the 66dp safe circle -->
+  <g transform="translate(54,54) scale(0.8) translate(-48,-48)">
+    <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>
+    <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>
+  </g>
+</svg>