v0.3.1

CownCapsule serialization support for nested cowns.

Bug Fixes

• Removed the ownership check in _cown_shared that prevented a
  CownCapsule from being serialized to XIData when it was the value
  of another Cown. The check was unnecessary — _cown_shared only
  stores a pointer and ownership is enforced at acquire time.

Improvements

• Added CownCapsule.__reduce__ with COWN_INCREF pinning so that a
  CownCapsule embedded in a container (dict, list, etc.) can survive
  the pickle round-trip used by object_to_xidata. A module-level
  reconstructor (_cown_capsule_from_pointer) inherits the pin without
  a redundant COWN_INCREF, and validates the process ID on unpickle to
  guard against cross-process misuse.