```ts
/** Sifts events into lanes by channel prefix and severity ceiling. */
export function siftEventStreams(
  lanes: Array<Record<string, unknown>>,
  events: Array<Record<string, unknown>>,
): Record<string, unknown> {
  const RANK = ["chatter", "notice", "alarm", "panic"];
  if (!Array.isArray(lanes) || !Array.isArray(events)) {
    throw new Error("Lanes and events must be arrays.");
  }
  const gathered: { name: string; took: number[] }[] = lanes.map((lane: any) => ({
    name: lane.name as string,
    took: [] as number[],
  }));
  if (gathered.some(lane => typeof lane.name !== "string" || lane.name.trim() === "")) {
    throw new Error("Lane names must be non-empty strings.");
  }
  const dropped: number[] = [];
  events.forEach((event: any, at: number) => {
    if (
      !event ||
      typeof event.channel !== "string" ||
      event.channel.trim() === "" ||
      !RANK.includes(event.severity)
    ) {
      throw new Error("Event must have a non-empty string channel and one of the four severities.");
    }
    let caught = false;
    lanes.forEach((lane: any, which: number) => {
      if (!event.channel.startsWith(lane.prefix)) return;
      if (RANK.indexOf(event.severity) >= RANK.indexOf(lane.upTo)) return;
      gathered[which].took.push(at);
      caught = true;
      if (lane.last) return;
    });
    if (!caught) dropped.push(at);
  });
  return { lanes: gathered, dropped };
}
```