mapper/batcher: restructure internals for correctness

Move per-node pending changes from a shared xsync.Map on the batcher
into multiChannelNodeConn, protected by a dedicated mutex. The new
appendPending/drainPending methods provide atomic append and drain
operations, eliminating data races in addToBatch and
processBatchedChanges.

Add sync.Once to multiChannelNodeConn.close() to make it idempotent,
preventing panics from concurrent close calls on the same channel.

Add started atomic.Bool to guard Start() against being called
multiple times, preventing orphaned goroutines.

Add comprehensive concurrency tests validating these changes.

hscontrol/util/race.go

@@ -0,0 +1,6 @@
+//go:build race
+
+package util
+
+// RaceEnabled is true when the race detector is active.
+const RaceEnabled = true