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/0001-event-driven-cqrs.md

@@ -0,0 +1,23 @@
+# 0001 — Event-driven CQRS
+
+**Status:** accepted
+**Date:** 2026-06-18
+
+## Context
+
+K-Frame has a natural write/read split: the write model is config (widgets, data sources, layout) mutated via web UI, the read model is runtime state (current widget data, active layout) pushed to display clients. We need a mechanism to bridge the two — when config changes, runtime behavior must update (restart polling loops, push new layout to clients).
+
+## Decision
+
+Full event-driven CQRS. Commands mutate config and emit domain events. The read side projects current state from events and DataSource poll results. Events drive side effects: restarting poll loops, pushing updates to connected clients.
+
+## Alternatives considered
+
+**Imperative approach** — commands mutate config, then application layer imperatively calls "restart poll loop" / "push to clients." Simpler, but tangles command handling with side-effect orchestration. Adding new reactions to config changes requires modifying command handlers.
+
+## Consequences
+
+- Clean separation: command handlers only mutate config and emit events, never trigger side effects directly.
+- New reactions to config changes are just new event listeners — no command handler modification.
+- More moving parts than the imperative approach — event bus, event handlers, projections.
+- Events are in-memory only (no event store / event sourcing). This is CQRS, not ES.