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/test_helpers_test.go

@@ -40,7 +40,7 @@ func createMockServer() *Server {
 	s := &Server{
 		logger:       logger,
 		erupeConfig:  &cfg.Config{},
-		stages:       make(map[string]*Stage),
+		// stages is a StageMap (zero value is ready to use)
 		sessions:     make(map[net.Conn]*Session),
 		handlerTable: buildHandlerTable(),
 		raviente: &Raviente{