defineMod

The entry point of the SDK. A mod's module default-exports one `defineMod` call and nothing else. The returned value is data. It registers nothing, starts nothing, and touches no host state. The host folds it twice. At build time it runs the module in a throwaway sandbox and reads the value to derive the serializable manifest. At runtime it folds the same value again to wire your handlers into their lanes: ext-host for commands, tools, effects, and hooks; the sandboxed renderer for views and widgets. That double fold is why the module must be pure. It is re-executed to produce the manifest, so anything at the top level, a file read, a network call, a timer, a `console` line, runs during a build with no host attached. Put behavior inside handlers: `run`, `activate`, `handle`. The `ValidateModHooks` constraint makes the call fail to typecheck when a `hook.beforeAction` targets an action that is neither a known host action nor `command:` plus one of this mod's own command ids.