docs(assets): update celestial banner with optical centering and generator script

- Rewrite generate_banner.py with Pillow supersampling, Montserrat font caching, seamless background extension, and ambient glow
- Add horizontal offset (--offset-x, default +160px) to balance visual weight against the crescent moon
- Add raw background asset artemis-banner-raw.jpg
- Regenerate and synchronize artemis-banner.png, artemis-banner-cn.png, and artemis-banner-en.png
- Bump banner image cache version in README.md and README_CN.md

README.md

@@ -1,5 +1,5 @@
 <p align="center">
-  <img src="./docs/assets/artemis-banner.png?v=6" alt="ARTEMIS Banner" width="100%" />
+  <img src="./docs/assets/artemis-banner.png?v=7" alt="ARTEMIS Banner" width="100%" />
 </p>
 
 <p align="center">