[core] Document EventPool sizing requirement with LockFreeQueue

Add a comment to EventPool documenting that when paired with a
LockFreeQueue<T, N>, the pool should be sized to N-1 (the queue's
actual capacity) to prevent slot leaks and SPSC violations.

esphome/core/event_pool.h

@@ -13,6 +13,13 @@ namespace esphome {
 // Events are allocated on first use and reused thereafter, growing to peak usage
 // @tparam T The type of objects managed by the pool (must have a release() method)
 // @tparam SIZE The maximum number of objects in the pool (1-255, limited by uint8_t)
+//
+// SIZING: When paired with a LockFreeQueue<T, Q_SIZE>, the pool SIZE should be
+// Q_SIZE - 1 (the queue's actual capacity, since the ring buffer reserves one slot).
+// This ensures allocate() returns nullptr before push() can fail, which:
+//  - Prevents permanently leaking a pool slot
+//  - Avoids needing release() on the producer path after a failed push(),
+//    preserving the SPSC contract on the internal free list
 template<class T, uint8_t SIZE> class EventPool {
  public:
   EventPool() : total_created_(0) {}