```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"];

  // Validate input
  if (!Array.isArray(lanes) || !Array.isArray(events)) {
    throw new Error("Lanes and events must be arrays.");
  }

  lanes.forEach((lane: any) => {
    if (
      typeof lane.name !== "string" ||
      lane.name.trim() === "" ||
      typeof lane.prefix !== "string" ||
      RANK.indexOf(lane.upTo as string) === -1 ||
      typeof lane.last !== "boolean"
    ) {
      throw new Error("Invalid lane properties.");
    }
  });

  events.forEach((event: any) => {
    if (
      typeof event.channel !== "string" ||
      event.channel.trim() === "" ||
      RANK.indexOf(event.severity as string) === -1
    ) {
      throw new Error("Invalid event properties.");
    }
  });

  const gathered = lanes.map((lane: any) => ({
    name: lane.name as string,
    took: [] as number[],
  }));
  const dropped: number[] = [];

  events.forEach((event: any, at: number) => {
    let caught = false;
    for (const { which, lane } of lanes.map((lane, index) => ({ which: index, lane }))) {
      if (!event.channel.startsWith(lane.prefix)) continue;
      if (RANK.indexOf(event.severity as string) > RANK.indexOf(lane.upTo as string)) continue;
      gathered[which].took.push(at);
      caught = true;
      if (lane.last === true) break;
    }
    if (!caught) dropped.push(at);
  });

  return { lanes: gathered, dropped };
}
```