build-game

/build-application — a game is an application. Map game concepts to feature layers: data/ = value types, rules math, components, resources (the board, the player, the score) and entities (the pieces); spec/ = the game state + moves (+ one pure function per system); indexes = spatial/lookup queries; transactions + actions = moves/spawns; computed = score/status/winner; ui = view + input. - **Turn-based** uses the reactive transaction → computed → ui loop, no systems (see data-lit-tictactoe). - **Real-time** adds the `build-systems` phase: a per-frame tick loop (movement, collision, lifetime) driven by the scheduler, over many entities in distinct archetypes. Systems must stay O(1) per touched entity — never project the whole `State` into/out of the store each frame, and never turn per-row work into per-entity transactions. Each system conforms alone and as a frame (see `features/ecs/systems.md`). This is where components, archetypes, indexes, and systems all carry real weight.