add domain glossary, design spec, and ADRs

CONTEXT.md: domain model with entities (WidgetConfig, DataSource, LayoutPreset),
value objects (Layout, LayoutNode, KeyMapping, WidgetState, Sizing),
architecture decisions (CQRS, static dispatch, postcard), client domain model,
and design constraints.

Cargo.toml

@@ -0,0 +1,5 @@
+[workspace]
+members = []
+resolver = "2"
+
+[workspace.dependencies]