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.

docs/adr/0002-static-dispatch-over-trait-objects.md

@@ -0,0 +1,23 @@
+# 0002 — Static dispatch over trait objects
+
+**Status:** accepted
+**Date:** 2026-06-18
+
+## Context
+
+Hexagonal architecture uses port traits extensively. Rust offers two ways to consume them: trait objects (`Box<dyn Port>`, dynamic dispatch) or generics (`T: Port`, static dispatch / monomorphization).
+
+## Decision
+
+Use generics for all port traits. No `Box<dyn>` anywhere. Composition roots (bootstrap on server, firmware crate on client) are the only places that know concrete types. Generic parameters propagate through application and presentation layers.
+
+## Alternatives considered
+
+**Trait objects** — simpler type signatures, less generic noise. But adds vtable indirection, heap allocation per trait object, and prevents compiler optimizations. On ESP32 with limited RAM, every allocation matters.
+
+## Consequences
+
+- Zero-cost abstractions — port calls are monomorphized, inlined where possible.
+- More verbose type signatures — structs and functions carry generic parameters.
+- Compile times may increase due to monomorphization.
+- Consistent approach across server and client codebases.