refactor(channelserver): replace global stagesLock with sync.Map-backed StageMap

The global stagesLock sync.RWMutex protected map[string]*Stage, causing
all stage operations to contend on a single lock even for unrelated
stages. Any stage creation or deletion blocked all reads server-wide.

Replace with a typed StageMap wrapper around sync.Map which provides
lock-free reads and allows concurrent writes to disjoint keys. Per-stage
sync.RWMutex remains unchanged for protecting individual stage state.

StageMap exposes Get, GetOrCreate, StoreIfAbsent, Store, Delete, and
Range methods. Updated ~50 call sites across 6 production files and
9 test files.

server/channelserver/handlers_coverage3_test.go

@@ -984,7 +984,7 @@ func TestHandleMsgSysCreateStage_Coverage3(t *testing.T) {
 		default:
 			t.Error("no response queued")
 		}
-		if _, exists := server.stages["test_create_stage"]; !exists {
+		if _, exists := server.stages.Get("test_create_stage"); !exists {
 			t.Error("stage should have been created")
 		}
 	})