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/0003-postcard-over-flatbuffers.md

@@ -0,0 +1,26 @@
+# 0003 — postcard over FlatBuffers
+
+**Status:** accepted
+**Date:** 2026-06-18
+
+## Context
+
+Need a binary serialization format for the TCP protocol between server and ESP32 clients. Must be compact (bandwidth-constrained), ideally `no_std` compatible for future bare-metal targets.
+
+## Decision
+
+Use `postcard` with `serde` derives instead of FlatBuffers.
+
+## Alternatives considered
+
+**FlatBuffers** — zero-copy reads, schema-driven. But requires external codegen toolchain (flatc), `no_std` support is limited, and zero-copy is overkill for our small messages (a handful of widgets).
+
+**Custom binary protocol** — maximum control, zero deps. But significant implementation effort for serialize/deserialize code, error-prone, and postcard already solves this.
+
+## Consequences
+
+- `no_std` protocol crate out of the box — future-proof for bare-metal targets.
+- No codegen step — types are normal Rust structs with serde derives.
+- Compact wire format via varint encoding.
+- No zero-copy reads — messages are deserialized into owned types. Acceptable for our message sizes.
+- Adds `serde` and `postcard` as dependencies to the protocol crate.