Document single-primary active-turn nesting limit

Concurrent multi-observer setups overwrite the process-wide turn
parent used by permission.wait and subagent spans.

Author: TheGreatAxios

src/perf/active-turn.ts

@@ -1,7 +1,10 @@
 /**
  * Process-wide open-turn id for nesting permission.wait / subagent outside the
- * reactor observer. Single-primary assumption: one run-sink observer owns the
- * slot. `clear()` and observer `reset()`/`closeTurn` null it.
+ * reactor observer.
+ *
+ * Single-primary assumption: one run-sink observer owns the slot. A second
+ * concurrent observer overwrites the parent used by gate/task spans — not
+ * supported. `clear()`, observer `reset()`, and `closeTurn` null the slot.
  */
 
 let activeTurnId: string | null = null;