Build Application-owned widget selection

type State = "draft" | "widget_open" | "fallback_active" | "candidate_received" | "selected" | "submitted" | "cancelled";
type CommandName = "create_session" | "open_widget" | "activate_fallback" | "receive_candidate" | "accept_selection" | "edit_host_text" | "submit" | "cancel";

type Command = {
  name: CommandName;
  aggregateId: string;
  expectedVersion: number;
  idempotencyKey: string;
};

const transitions = {
  "create_session": { from: [null], to: "draft", event: "host.session_created" },
  "open_widget": { from: ["draft", "fallback_active"], to: "widget_open", event: "widget.opened" },
  "activate_fallback": { from: ["widget_open"], to: "fallback_active", event: "widget.fallback_activated" },
  "receive_candidate": { from: ["widget_open", "fallback_active"], to: "candidate_received", event: "location.candidate_received" },
  "accept_selection": { from: ["candidate_received"], to: "selected", event: "location.selected" },
  "edit_host_text": { from: ["draft", "widget_open", "fallback_active", "candidate_received", "selected"], to: "draft", event: "host.text_edited" },
  "submit": { from: ["selected"], to: "submitted", event: "host.submitted" },
  "cancel": { from: ["draft", "widget_open", "fallback_active", "candidate_received", "selected"], to: "cancelled", event: "host.cancelled" },
} as const;

export function decide(current: { state: State | null; version: number }, command: Command) {
  const rule = transitions[command.name];
  if (command.expectedVersion !== current.version) throw new Error("version_conflict");
  if (!rule.from.includes(current.state as never)) throw new Error("invalid_transition");
  return {
    state: rule.to as State,
    version: current.version + 1,
    event: rule.event,
    idempotencyKey: command.idempotencyKey,
  };
}

// Persist the result, immutable event, audit row, and outbox intent atomically.
// Store the first result by idempotencyKey before executing another effect.