Authoring guide for TypeScript app cores - Model, Msg, update, and the pure functions they call, written in the closed app-core subset and compiled ahead-of-time to arena-backed Zig by the @native-sdk/core transpiler. Use when writing or modifying a src/core.ts app core, fixing subset checker errors (NS1001-NS1060), or deciding how to express state, messages, text (bytes and the byte-text string methods), text input, continuous controls (sliders, scroll), effects (Cmd), subscriptions (Sub), the host-event wiring channels (frameMsg, keyMsg, appearanceMsg, chromeMsg, envMsgs, app.zon assets), derived values, the view_unbound lint opt-out, local mutation of owned arrays, or how to split a core into modules under src/ (relative imports, namespace imports, @native-sdk/core/text, @native-sdk/core/events).