Improvements:

- Added CownCapsule.disown() — abandons a cown's value without
  serializing it and resets ownership to NO_OWNER. Used during worker
  cleanup to safely discard orphan cowns before the owning interpreter
  is destroyed, preventing dangling Python object references.
- Rewrote receive to use a two-phase spin-then-park strategy for
  single-tag untimed receives. Phase 1 spins for BOC_SPIN_COUNT
  iterations; Phase 2 parks the thread on a per-queue condvar, eliminating
  busy-wait CPU burn. Timed receives and multi-tag receives use
  spin-then-backoff with exponential sleep (1 µs → 1 ms cap).
- Added platform-abstracted condvar primitives (BOCParkMutex /
  BOCParkCond) with implementations for Windows (SRWLOCK /
  CONDITION_VARIABLE), macOS (pthreads), and Linux (C11 threads).
- Each BOCQueue now carries a waiters counter, park_mutex, and
  park_cond. Producers signal parked receivers after enqueue;
  drain and set_tags broadcast to wake all parked threads.
- Replaced the fixed thrd_sleep in send with a sched_yield /
  SwitchToThread, reducing send-side latency.
- Refactored the monolithic _core_receive into receive_single_tag
  and receive_multi_tag, each with its own backoff/parking logic.
- Moved the BOC_QUEUE_DISABLED check earlier in get_queue_for_tag
  so callers skip disabled queues instead of returning NULL after
  tag resolution.
- Added Windows-compatible atomic_load_explicit /
  atomic_fetch_add_explicit / atomic_fetch_sub_explicit macros
  using InterlockedExchangeAdd64.
- Declared Py_mod_gil = Py_MOD_GIL_NOT_USED in both _core and
  _math C extensions so that importing bocpy on a free-threaded
  Python build (3.13t+) does not re-enable the GIL.
- Replaced PyDict_GetItem (borrowed reference) with
  PyDict_GetItemRef (strong reference) in BOCRecycleQueue_recycle
  on Python 3.13+, improving forward-compatibility with free-threaded
  builds.

Bug Fixes:

- Fixed a deadlock when the same cown is passed multiple times to @when
  (e.g. @when(c, c)). Duplicate requests for the same cown caused the
  MCS-queue-based two-phase locking to spin-wait on itself. Requests are
  now deduplicated by target cown in Behavior.__init__, with
  compensating resolve_one calls to maintain the behavior count
  invariant.

Tests:

- TestLostWakeStress: single-producer random delays, bursty producer,
  and repeated single-message wake to detect lost-wake races.
- TestMultiTagBackoff: multi-tag receive correctness — second-tag hit,
  delayed arrival, per-tag FIFO ordering, timeout, and interleaved
  producers.
- TestTimeoutAccuracy: lower-bound / upper-bound wall-clock checks and
  zero-timeout immediacy.
- Added tests for duplicate cowns in @when: same cown twice, thrice,
  non-adjacent duplicates, duplicates within a group, and mutation
  aliasing semantics.

CI:

- Added a free-threaded CI job that tests against Python 3.13t and
  3.14t on Linux, with explicit assertions that the GIL remains disabled
  after import.

Signed-off-by: Matthew A Johnson <matjoh@microsoft.com>