```ts
/** Tape-measure spans: parse to inches, format canonically, and add. */

const INCHES: Record<string, number> = { yd: 36, ft: 12, in: 1 };
const ORDER = ["yd", "ft", "in"];

export function parseSpan(text: string): number {
  if (typeof text !== "string" || text.trim() === "") {
    throw new Error("span must be a non-empty string");
  }

  const parts = text.split(/\s+/).filter(Boolean);
  let total = 0;

  for (const part of parts) {
    const match = part.match(/^(\d+)yd$/i);
    if (match) {
      total += parseInt(match[1], 10) * INCHES.yd;
      continue;
    }

    match = part.match(/^(\d+)ft$/i);
    if (match) {
      total += parseInt(match[1], 10) * INCHES.ft;
      continue;
    }

    match = part.match(/^(\d+)in$/i);
    if (match) {
      total += parseInt(match[1], 10) * INCHES.in;
      continue;
    }

    throw new Error("unknown unit in span");
  }

  return total;
}

export function formatSpan(inches: number): string {
  if (!Number.isInteger(inches) || inches < 0) {
    throw new Error("inches must be a non-negative integer");
  }
  if (inches === 0) {
    return "0in";
  }
  const parts: string[] = [];
  const yards = Math.floor(inches / 36);
  const feet = Math.floor((inches % 36) / 12);
  const rest = inches % 12;
  if (yards > 0) {
    parts.push(`${yards}yd`);
  }
  if (feet > 0) {
    parts.push(`${feet}ft`);
  }
  if (rest > 0) {
    parts.push(`${rest}in`);
  }
  return parts.join(" ");
}

export function addSpans(first: string, second: string): string {
  const total = parseSpan(first) + parseSpan(second);
  return formatSpan(total);
}
```