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-background.svg

@@ -0,0 +1,10 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 108 108">
+  <!-- Android adaptive-icon background layer: full-bleed, launcher applies the mask -->
+  <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="108" height="108" fill="url(#tile)"/>
+</svg>