CngxAsyncBoundary

Headless boundary that aggregates N keyed CngxAsyncStates into one derived CngxAsyncState and provides it as CNGX_STATEFUL.

The aggregate state is a first-class CngxAsyncState, so it renders through <cngx-async-container [state]="b.state"> and every other consumer unchanged, and any transition bridge (cngxToastOn, cngxAlertOn, cngxBannerOn) nested in the host fires on the combined status with zero [state] wiring. failures exposes the errored sources - keyed, so a consumer's @for can render per-source attribution through any feedback component. state.error stays the first error for the single-error path.

<div [cngxAsyncBoundary]="sources" #b="cngxAsyncBoundary">
  <cngx-async-container [state]="b.state"> ... </cngx-async-container>
  <cngx-toast-on />
  @for (f of b.failures(); track f.key) {
    <cngx-banner>{{ f.label }} failed</cngx-banner>
  }
</div>