docs: add agy-generated README banner

Vibrant rainbow banner (rainbow ribbons + glow accents) evoking the
diversity of analysis methods, generated via agy/gemini-3.5-pro. Add
assets/banner.svg (XML-valid, foreground inside safe zone). Banner img
at top of README.md and README_de.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: Lukas Geiger

README.md

@@ -1,3 +1,7 @@
+<p align="center">
+  <img src="assets/banner.svg" width="100%" alt="MethodenAnalyser — Static Python analysis across a spectrum of methods">
+</p>
+
 <p align="center">
   <img src="https://img.shields.io/badge/Version-3.0-blue?style=for-the-badge" alt="Version">
   <img src="https://img.shields.io/badge/Python-3.10+-yellow?style=for-the-badge" alt="Python">

README_de.md

@@ -1,3 +1,7 @@
+<p align="center">
+  <img src="assets/banner.svg" width="100%" alt="MethodenAnalyser — Statische Python-Analyse über ein Spektrum von Methoden">
+</p>
+
 <p align="center">
   <img src="https://img.shields.io/badge/Version-3.0-blue?style=for-the-badge" alt="Version">
   <img src="https://img.shields.io/badge/Python-3.10+-yellow?style=for-the-badge" alt="Python">