```ts
/** A block-allocation bitmap: 32-bit words, LSB-first, a set bit is occupied. */
const WORD_BITS = 32;
const WORD_MAX = 0xffffffff;

function checkWords(words: number[]): void {
  for (const word of words) {
    if (!Number.isInteger(word) || word < 0 || word > WORD_MAX) {
      throw new Error("bitmap words must be integers from 0 to 4294967295");
    }
  }
}

function bitOf(words: number[], index: number): number {
  return (words[Math.floor(index / WORD_BITS)] >>> index % WORD_BITS) & 1;
}

export function bitAt(words: number[], index: number): number {
  checkWords(words);
  if (!Number.isInteger(index)) {
    throw new Error("bit index must be an integer");
  }
  if (index < 0 || index >= words.length * WORD_BITS) {
    throw new Error("bit index is outside the words");
  }
  return bitOf(words, index);
}

export function occupyRun(words: number[], start: number, length: number): number[] {
  checkWords(words);
  if (!Number.isInteger(start) || start < 0) {
    throw new Error("run start must be a non-negative integer");
  }
  if (!Number.isInteger(length) || length <= 0) {
    throw new Error("run length must be a positive integer");
  }
  if (start + length > words.length * WORD_BITS) {
    throw new Error("run leaves the words");
  }
  for (let i = start; i < start + length; i++) {
    if (bitOf(words, i) === 1) {
      throw new Error("run touches an occupied block");
    }
  }
  const marked = words.slice();
  for (let i = start; i < start + length; i++) {
    const word = Math.floor(i / WORD_BITS);
    marked[word] = (marked[word] | (1 << i % WORD_BITS)) >>> 0;
  }
  return marked;
}

export function findFreeRun(words: number[], bits: number, length: number): number {
  checkWords(words);
  if (!Number.isInteger(bits) || bits <= 0) {
    throw new Error("capacity must be a positive integer");
  }
  const requiredWords = Math.ceil(bits / WORD_BITS);
  if (words.length !== requiredWords) {
    throw new Error("word count does not match capacity");
  }
  if (bitOf(words, bits) === 1) {
    throw new Error("extra bit beyond capacity is set");
  }

  for (let start = 0; start <= bits - length; start++) {
    let free = true;
    for (let i = start; i < start + length; i++) {
      if (bitOf(words, i) === 1) {
        free = false;
        break;
      }
    }
    if (free) {
      return start;
    }
  }

  return -1;
}
```