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/wordmark-on-dark.svg

@@ -0,0 +1,17 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="-10 -92 433 104">
+  <!-- "echolot" — hand-drawn monoline letterforms (no font dependency). For dark grounds. -->
+  <g fill="none" stroke="#E8F4F2" stroke-width="11" stroke-linecap="round">
+    <path d="M 0 -24 H 48"/>
+    <path d="M 48 -24 A 24 24 0 1 0 40.97 -7.03"/>
+    <path d="M 110.97 -40.97 A 24 24 0 1 0 110.97 -7.03"/>
+    <path d="M 140 -76 V 0"/>
+    <path d="M 140 -24 A 24 24 0 0 1 188 -24 L 188 0"/>
+    <circle cx="234" cy="-24" r="24"/>
+    <path d="M 280 -76 V 0"/>
+    <circle cx="326" cy="-24" r="24" stroke="#35E0C4"/>
+    <path d="M 372 -48 H 402"/>
+    <path d="M 387 -68 V 0"/>
+  </g>
+  <!-- the finding -->
+  <circle cx="326" cy="-24" r="6.5" fill="#FFB454"/>
+</svg>