[core] Replace scheduler pool vector with unbounded intrusive freelist (#16172)

tests/benchmarks/core/bench_scheduler.cpp

@@ -101,8 +101,8 @@ static void Scheduler_SetTimeout(benchmark::State &state) {
   Component dummy_component;
 
   // Register 3 timeouts then call() — realistic worst case where multiple
-  // components schedule in the same loop iteration. Keeps item count within
-  // the recycling pool (MAX_POOL_SIZE=5) to avoid spurious malloc/free.
+  // components schedule in the same loop iteration. warm_pool fills the
+  // freelist so acquire/recycle never falls back to malloc.
   static constexpr int kBatchSize = 3;
   static_assert(kInnerIterations % kBatchSize == 0, "kInnerIterations must be divisible by kBatchSize");
   warm_pool(scheduler, &dummy_component, kBatchSize, 1000);
@@ -209,9 +209,9 @@ static void Scheduler_SetTimeout_ExceedPool(benchmark::State &state) {
   Scheduler scheduler;
   Component dummy_component;
 
-  // Register 10 timeouts then call() — exceeds MAX_POOL_SIZE=5 to measure
-  // the performance cliff when the recycling pool is exhausted and items
-  // must be malloc'd/freed.
+  // Register 10 timeouts then call() — larger working set than the 3-item
+  // batches above. With the unbounded freelist, warm_pool preallocates 10
+  // items so this measures steady-state, not malloc cliff.
   static constexpr int kBatchSize = 10;
   static_assert(kInnerIterations % kBatchSize == 0, "kInnerIterations must be divisible by kBatchSize");
   warm_pool(scheduler, &dummy_component, kBatchSize, 1000);