docs: redesign banners with Cormorant Garamond + 2x supersample

Dark banner:
- Replace Georgia with Cormorant Garamond SemiBold (actual brand font)
- 2x supersample render → Lanczos downsample for crisp edges at all
  display densities
- Shell (#141d18) background with layered radial warmth
- Editorial stacked colophon layout: large 'A' / thin parchment rule /
  wide-tracked wordmark / amber italic tagline
- Ghost full-width hairline rule anchors the horizontal composition
- Terracotta dash ornaments flank the tagline (composited on separate
  RGBA layer for correct alpha blending)

Light banner (new PNG, replacing SVG):
- Same font, supersample, and layout
- Paper (#f5f0e7) background with subtle warm edge shadow
- Ink wordmark, Muted tagline — all brand palette

README: switch light-mode fallback from banner-light.svg to banner-light.png

docs/README.md

@@ -1,7 +1,7 @@
 <p align="center">
   <picture>
     <source media="(prefers-color-scheme: dark)" srcset="branding/assets/banner-dark.png">
-    <img alt="Archivr — Preserve what matters. Forever." src="branding/assets/banner-light.svg" width="860">
+    <img alt="Archivr — Preserve what matters. Forever." src="branding/assets/banner-light.png" width="860">
   </picture>
 </p>