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>

README.md

@@ -1,3 +1,7 @@
+<p align="center">
+  <img src="assets/branding/banner.svg" alt="echolot — measure, don't guess" width="100%">
+</p>
+
 # Echolot
 
 Free software for detecting and debugging **local network issues** from an Android phone —