Add raster panel icon family

design/brand/panel-variants/README.md

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+# Burrow Raster Panel Icon Variants
+
+The six PNG masters here are cropped from `source-sheet.png` and are the
+locked app icon source. Do not redraw them; regenerate package assets from
+these rasters.
+
+| Variant | Use | Source |
+| --- | --- | --- |
+| `panel-01-guardian` | iOS primary, Android primary, Linux primary | Guardian, top-left panel |
+| `panel-02-field` | iOS alternate | Field, top-middle panel |
+| `panel-03-burrow` | iOS alternate, watchOS primary | Burrow, top-right panel |
+| `panel-04-sentinel` | iOS alternate | Sentinel, bottom-left panel |
+| `panel-05-stride` | iOS alternate, macOS primary | Stride, bottom-middle panel |
+| `panel-06-post` | iOS alternate | Post, bottom-right panel |
+
+`preview-strip.png` shows the six raster masters side by side.