Brand: integrate the MermaidKit logo (site favicon + header + hero, README)

Adds the logo — a deep-teal tile with an M in the coral→seafoam gradient over
a lavender wave, drawn in the exact mermaid palette. Full brand/app-icon source
set (per-platform SVGs, macOS .icns + .iconset, PNG renders 16–1024) lives in
assets/; the web subset (logo.svg, logo-256/512.png) in docs/images/.

Site: SVG favicon (+ PNG and apple-touch fallbacks), a 26px mark beside the
header wordmark, and an 84px rounded logo above the hero headline. README:
centered logo above the title.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JoLDcosyaHg3tAKhU5SQaw

Author: clintecker

README.md

@@ -1,3 +1,7 @@
+<p align="center">
+  <img src="docs/images/logo-256.png" alt="MermaidKit" width="120" height="120">
+</p>
+
 # MermaidKit
 
 Native [Mermaid](https://mermaid.js.org) diagrams in pure Swift — no