[core] Replace scheduler pool vector with unbounded intrusive freelist

The fixed MAX_POOL_SIZE=5 cap was the source of the heap churn the pool was
meant to prevent: any device with more than 5 concurrent timers (e.g. a board
with 30+ LD2450 sensors) hit a steady-state oscillation of recycle->delete and
acquire->new on every loop iteration.

Replace std::vector<SchedulerItem*> with a singly-linked freelist threaded
through SchedulerItem::next_free, which shares storage with `component` via an
anonymous union (zero per-item overhead -- the component pointer is dead while
pooled). Drop the cap entirely: the freelist quiesces at the application's
natural concurrent-timer high-water mark, which is the working set the device
already needs while those timers are active.

No std::vector means no growth-doubling slack and no realloc copies during
warm-up. Caller of get_item_from_pool_locked_() must overwrite item->component
before unlocking (already true at the sole call site); nullptr remains a valid
live `component` value for SELF_POINTER items, so we cannot pre-clear it.

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);