Hold host calls inside the bridge so the quiesce is tested, not assumed

The closerace scenario pinned the order of the count and the notify but never
that a host call is counted at all. Deleting the call scope from
`cultmesh_quic_next_event` outright leaves every configuration green — the
pollers finish before the teardown reaches them — and so does inverting the
comparison that makes a positive timeout block, which empties the library before
the close even begins. A scenario that only sleeps and hopes cannot tell any of
that apart from the real thing.

So the bridge says it itself, in development builds only (section 7 of the
header, behind CULTMESH_QUIC_DEBUG_ASSERTS, absent from every shipped binary):
the peak number of host calls counted inside the library, the number counted
when the close began its wait, and a hold that parks a call inside the library
until the scenario releases it. A held call returns the 0 an empty queue would
have and touches the runtime no further, so a bridge that frees the runtime
under it is reported by the scenario rather than crashing inside it.

`holdclose` drives the quiesce through that seam: the calls are counted, they
were still counted when the close began, and the close did not return while they
were held. `closerace` keeps hunting the race and now also refuses to pass when
a poller left before the close began, which is a scenario that raced nothing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

native/GameCult.Mesh.Quic.Native/include/cultmesh_quic_native.h

@@ -308,6 +308,38 @@ CULTMESH_QUIC_API int32_t cultmesh_quic_last_error(
  * when nothing has failed. */
 CULTMESH_QUIC_API int32_t cultmesh_quic_last_status(void* runtime);
 
+/* ---------------------------------------------------------------------------
+ * 7. The development seam
+ *
+ * Present only in a library configured with `CULTMESH_QUIC_DEBUG_ASSERTS`, which
+ * a shipped build never is. It exists for the runtime-lifetime scenarios: the
+ * quiesce is a promise about host calls that are inside the library, and a
+ * scenario that can only sleep and hope cannot tell a bridge that counts them
+ * from one that does not.
+ *
+ * It is process-wide, not per-runtime, because the numbers outlive the runtime
+ * they describe: `cultmesh_quic_runtime_close` frees the runtime, and the count
+ * it waited on is read afterwards. The scenarios open one runtime at a time.
+ * --------------------------------------------------------------------------- */
+#if defined(CULTMESH_QUIC_DEBUG_ASSERTS)
+
+/* Non-zero arms the hold and resets both counters: every call that reaches the
+ * blocking wait in `cultmesh_quic_next_event` parks inside the library instead
+ * of returning. Zero releases every held call and disarms it. A held call
+ * returns 0 and touches the runtime no further. */
+CULTMESH_QUIC_API void cultmesh_quic_debug_hold_calls(int32_t hold);
+
+/* The high-water mark of host calls counted inside the library since the hold
+ * was armed. Zero means no call was ever counted. */
+CULTMESH_QUIC_API int32_t cultmesh_quic_debug_peak_calls(void);
+
+/* How many host calls were counted inside the library when the last
+ * `cultmesh_quic_runtime_close` began its wait, or -1 if no close has begun one
+ * since the hold was armed. This is the number the wait exists for. */
+CULTMESH_QUIC_API int32_t cultmesh_quic_debug_calls_at_close(void);
+
+#endif /* CULTMESH_QUIC_DEBUG_ASSERTS */
+
 #if defined(__cplusplus)
 }
 #endif