createAnnouncementPhrase

Description#

Declarative live-region phrase with transition arming and expiry - the shared shape behind "voice the change, not the standing state". A real transition (arm) sets the phrase; an expiry transition (spend) clears an already-voiced phrase so a later content change cannot re-announce it; everything else keeps the previous phrase.

Fully derived (linkedSignal over source snapshots) - no effect, no imperative previous-value tracking.

Eager-read caveat. linkedSignal only observes source snapshots it is actually read under. A consumer that renders this phrase behind a short-circuiting arm (busy() ? busyPhrase : phrase()) MUST read the phrase eagerly before branching, otherwise the phrase never observes the busy-start snapshot and the spend transition is skipped.

private readonly selectionPhrase = createAnnouncementPhrase({
  source: () => ({ selected: this.selected(), loading: this.loading() }),
  arm: (curr, prev) =>
    curr.selected !== prev.selected ? (curr.selected ? 'Selected' : 'Deselected') : null,
  spend: (curr, prev) => curr.loading && !prev.loading,
});