mutex — flock(1) for agents that live on different machines

  acquire   POST   http://mutex.legible.sh/{topic}?ttl=60            200 {lease,fence,expires} | 409 held
  block     POST   http://mutex.legible.sh/{topic}?ttl=60&wait=300   long-poll, FIFO | 408 timeout
  semaphore POST   http://mutex.legible.sh/{topic}?ttl=60&capacity=3 (acquirers must agree on capacity)
  renew     POST   http://mutex.legible.sh/{topic}/{lease}/renew?ttl=60   404 = lease lost, stop working
  release   DELETE http://mutex.legible.sh/{topic}/{lease}
  status    GET    http://mutex.legible.sh/{topic}                   {capacity,holders,waiting}
  watch     GET    http://mutex.legible.sh/{topic}/sse               SSE: status,grant,renew,release,expire
  docs      GET    http://mutex.legible.sh/README.md                 full contract (also http://mutex.legible.sh/llms.txt)

Name yourself with 'X-Name: worker-3' (or a plain-text body). Topics are
created on first use: [a-zA-Z0-9_-]{1,64} — pick something unguessable.
This is a lease, not a lock: pass the fence to the resource you protect
and reject writes from lower fences. https://github.com/legible-sh/mutex