Function assertNever

assertNever(value: never): never
Exhaustiveness probe for a switch (msg.kind). Place at the default arm; the parameter type never is satisfied only when every variant has a matching case. Adding a tag to the union without a case arm is a compile error at this call site.

The runtime throw is belt-and-suspenders only — it cannot be reached when the type-check passes.
Parameters
- value: never
  The post-narrowed message that should be never.
Returns never
Throws
Error citing the unmatched message variant.
Example
```
function update(msg: CounterMsg, g: Graph): void {
  switch (msg.kind) {
    case 'inc': return g.commit('inc', tx => ...)
    case 'dec': return g.commit('dec', tx => ...)
    case 'set': return g.commit('set', tx => tx.set(n, msg.value))
    default:    return assertNever(msg)
  }
}
```
- Defined in msg.ts:253