Add CAC3 icon set + art/ folder.

Generated from a 1254×1254 transparent-background source at:
  cac3-master-1024.png (1024×1024 master)
  cac3-app-store.png   (1500×1500 for app store listings)

Plus standard web/app sizes:
  - cac3-{16,32,48,96,192,512,1024}.png (downsamples)
  - favicon.ico (multi-icon bundle: 16,32,48,96)
  - favicon-{16x16,32x32,48x48}.png
  - apple-touch-icon.png (180×180), apple-touch-icon-ipad.png (152×152)
  - android-foreground.png (432×432, transparent)
  - android-background.png (432×432, solid #0a0a0a)

See art/icons/README.md for the full mapping into planned source-tree
slots (src/qt/res/icons/, contrib/debian, splash, etc).

Design released under CC0 1.0 (public domain) for downstream remix.

art/icons/README.md

@@ -0,0 +1,65 @@
+# CAC3 Icon Set
+
+## Source
+Generated from `cac3-master-1024.png`, a 1024×1024 transparent-background
+recolored tribute to the classic California commemorative coin design.
+
+## Files at a glance
+
+| File | Size | Purpose |
+|---|---|---|
+| `cac3-master-1024.png` | 1024×1024 | Source of truth for all smaller sizes. Replace this if you regenerate the icon set. |
+| `cac3-app-store.png` | 1500×1500 | Apple App Store / Google Play store-listing size |
+| `cac3-512.png` | 512×512 | PWA / modern desktop |
+| `cac3-192.png` | 192×192 | Android / PWA medium |
+| `cac3-96.png` | 96×96 | Linux desktop, taskbar |
+| `cac3-48.png` | 48×48 | Toolbar |
+| `cac3-32.png` | 32×32 | Menu |
+| `cac3-16.png` | 16×16 | Status bar / smallest |
+| `favicon.ico` | 16/32/48/96 multi-icon | Browsers (`.ico` bundle) |
+| `favicon-16x16.png` | 16×16 | Browser favicon |
+| `favicon-32x32.png` | 32×32 | Browser favicon (HiDPI) |
+| `favicon-48x48.png` | 48×48 | Browser favicon (Windows) |
+| `apple-touch-icon.png` | 180×180 | iOS home screen |
+| `apple-touch-icon-ipad.png` | 152×152 | iPad home screen |
+| `android-foreground.png` | 432×432, transparent | Android adaptive icon foreground layer |
+| `android-background.png` | 432×432 solid `#0a0a0a` | Android adaptive icon background layer (dark) |
+
+## How to regenerate
+
+If you change the source master, regenerate everything from a single command:
+
+```bash
+SRC=cac3-master-1024.png
+for s in 16 32 48 96 192 512 1024; do
+  convert "$SRC" -resize ${s}x${s} cac3-${s}.png
+done
+convert cac3-16.png cac3-32.png cac3-48.png cac3-96.png favicon.ico
+convert "$SRC" -resize 180x180 apple-touch-icon.png
+convert "$SRC" -resize 152x152 apple-touch-icon-ipad.png
+convert "$SRC" -resize 432x432 android-foreground.png
+convert "$SRC" -resize 1500x1500 cac3-app-store.png
+```
+
+(ImageMagick 7+. For IM6 substitute `magick` for `convert`.)
+
+## Planned targets in this codebase
+
+Once the C++ source lands, these files are referenced in:
+
+| Target file (planned) | Which icon |
+|---|---|
+| `src/qt/res/icons/cac3.png` | `cac3-512.png` |
+| `src/qt/res/icons/cac3.ico` | `favicon.ico` |
+| `src/qt/res/icons/cac3@2x.png` | `cac3-1024.png` |
+| `src/qt/res/icons/cac3-testnet.png` | distinct from mainnet |
+| `src/qt/res/splash.png` (the boot splash) | new generative splash once shipped |
+| `contrib/debian/cac3d.desktop` | `cac3-256.png` |
+| `contrib/builder/bootstraps/qml/out/assets/` | `cac3-512.png` |
+
+## License
+The icon design is released under **CC0 1.0 Universal** (public domain
+dedication). Anyone may fork, recolor, or remix it for downstream CAC3
+implementations without attribution. The recoloring of the classic
+California commemorative coin design is a tribute, not a claim of
+authorship.