fix(channels): fix memory hazards in channel abstraction layer

Address 7 memory/architecture issues affecting long-running gateway
processes on embedded devices (<10MB RAM):

- Fix dispatcher busy-wait: remove select+default pattern that caused
  CPU spin after context cancellation; SubscribeOutbound handles ctx
  internally
- Add TTL janitor for typingStops/placeholders sync.Map entries to
  prevent unbounded accumulation when outbound paths fail
- Reduce queue buffers from 100 to 16 slots (~84% memory reduction)
- Optimize SplitMessage with index-based rune operations to reduce
  intermediate string/rune allocations
- Replace uuid.New() with atomic counter + random prefix for media
  scope IDs (eliminates per-call crypto/rand syscall)
- Lazy channel worker creation: defer goroutine+buffer allocation
  until channel.Start() succeeds

pkg/bus/bus.go

@@ -11,6 +11,8 @@ import (
 // ErrBusClosed is returned when publishing to a closed MessageBus.
 var ErrBusClosed = errors.New("message bus closed")
 
+const defaultBusBufferSize = 16
+
 type MessageBus struct {
 	inbound       chan InboundMessage
 	outbound      chan OutboundMessage
@@ -21,9 +23,9 @@ type MessageBus struct {
 
 func NewMessageBus() *MessageBus {
 	return &MessageBus{
-		inbound:       make(chan InboundMessage, 100),
-		outbound:      make(chan OutboundMessage, 100),
-		outboundMedia: make(chan OutboundMediaMessage, 100),
+		inbound:       make(chan InboundMessage, defaultBusBufferSize),
+		outbound:      make(chan OutboundMessage, defaultBusBufferSize),
+		outboundMedia: make(chan OutboundMediaMessage, defaultBusBufferSize),
 		done:          make(chan struct{}),
 	}
 }

pkg/bus/bus_test.go

@@ -67,7 +67,7 @@ func TestPublishInbound_ContextCancel(t *testing.T) {
 
 	// Fill the buffer
 	ctx := context.Background()
-	for i := 0; i < 100; i++ {
+	for i := 0; i < defaultBusBufferSize; i++ {
 		if err := mb.PublishInbound(ctx, InboundMessage{Content: "fill"}); err != nil {
 			t.Fatalf("fill failed at %d: %v", i, err)
 		}
@@ -194,7 +194,7 @@ func TestPublishInbound_FullBuffer(t *testing.T) {
 	ctx := context.Background()
 
 	// Fill the buffer
-	for i := 0; i < 100; i++ {
+	for i := 0; i < defaultBusBufferSize; i++ {
 		if err := mb.PublishInbound(ctx, InboundMessage{Content: "fill"}); err != nil {
 			t.Fatalf("fill failed at %d: %v", i, err)
 		}