Implement the reviewed UI icon system and durable review workflow (#853)

Thanks @Nathan-Dane.

docs/ui-icon-design-proposal.md

@@ -0,0 +1,8 @@
+# UI icon design — superseded proposal
+
+The format and style decisions are implemented. SVG masters, theme role mapping,
+isometric geometry and the owner's reviewed set are authoritative.
+
+Use [the binding style guide](../assets/icons/ui/STYLE_GUIDE.md) and
+[the maintained review workflow](../assets/icons/ui/README.md). The earlier
+undecided proposal must not be used to create a competing style or drawing pipeline.