feat(ui)!: move the widget tree into C++

A node was a Lua table of ~625 bytes, of which 21 keys pushed it over a
power-of-two hash boundary and eight were style copies inheritance had
splattered down from its parent. A 400 node screen cost ~250 KB and could not
coexist with wifi's buffers.

The tree now lives in src/ui/layout.h as a 16 byte struct in a flat arena, and
splits by lifetime: Node holds what hit testing and repainting need forever,
Spec holds what only measure/place read and is dropped when layout ends. Style
is sparse and resolved by walking parents, so a node naming no colours costs
nothing. Re-layout rebuilds from Lua rather than retaining the inputs.

    401 nodes:  8218 B steady, 21050 B peak
           Lua: ~250000 B steady

sdcard/lib/ui.lua stays the toolkit and keeps every constructor signature, but
returns integer handles: 627 lines to 374. Composition, the palette and custom
painters are still Lua on the SD card; only primitives now need a reflash.

BREAKING CHANGE: ui constructors return handles, not tables. Use
ui.setText(id, text) and keep per-node app data in a table keyed by id.

src/lua/bindings.h

@@ -19,6 +19,7 @@ LuaApp* app(lua_State* L);
 void bindApp(lua_State* L, LuaApp* owner);
 
 void registerGui(lua_State* L);
+void registerNode(lua_State* L);
 void registerSys(lua_State* L);
 void registerSettings(lua_State* L);
 void registerInput(lua_State* L);