Function defineMsgs

defineMsgs<Spec extends MsgSpec>(spec: Spec): MsgBuilder<Spec>
Build a typed variant-constructor record plus closed Msg union from a record of tag → payload? declarations. The same record shape pairs cleanly with createUpdate's record-of-handlers, so tags are declared once.
Type Parameters
- Spec extends MsgSpec
  The spec record; keys are tags, values are null (no payload) or payload markers.
Parameters
- spec: Spec
  The variant spec.
Returns MsgBuilder<Spec>
A MsgBuilder whose entries are variant constructors.
Remarks
Adding a tag to spec widens MsgOf<typeof builder>. Passing that widened union to createUpdate<MsgOf<...>> makes the missing handler a compile error at the call site, and a missing arm in a switch (msg.kind) is a compile error at assertNever.
Example
```
const msg = defineMsgs({
  inc: null,
  dec: null,
  set: payload<{ value: number }>(),
})
type CounterMsg = MsgOf<typeof msg>

msg.inc()             // { kind: 'inc' }
msg.set({ value: 3 }) // { kind: 'set'; value: 3 }
```
- Defined in msg.ts:204