v0.6.1

Fixed

• spawn no longer fails when the machine is briefly out of PTY devices.
  On macOS a PTY is torn down with revoke() and its device recycled, and a
  suite asking for devices faster than the kernel returns them gets ENXIO —
  "Device not configured", which reads like a broken machine and is really a
  queue. cargo test runs one test per core by default, so this is what a
  sixteen-core Mac does with any suite of this shape; the failure was not
  exotic, it was Tuesday. openpty is now retried for about 1.6 seconds
  before giving up, releasing the PTY lifecycle lock between attempts —
  that lock is the one a teardown also takes, so waiting under it would have
  blocked the only work capable of freeing a device.

  Found by the stress workflow the first time it ran the suite at sixteen
  threads, on macOS; Linux had run the same suite twenty-five times over
  without noticing. tests/concurrency.rs now applies the same pressure on
  purpose — two dozen terminals at once, and eight rounds of open-and-recycle
  — so it is reproducible rather than a matter of which shard drew the short
  straw.