Fix daemon shutdown hang and concurrent-send race

- daemon_net: close the client transport on context cancellation so the
  per-connection Recv loop unblocks; otherwise wg.Wait() in the accept loop
  hung on a still-connected client and the daemon never stopped.
- protocol: guard ConnTransport.Send with a mutex so the subscriber pump and
  command handlers can push frames concurrently without racing the bufio.Writer.

Fixes TestDaemonDetachReattachPreservesProcess (now passes under -race).

internal/protocol/transport.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"io"
 	"net"
+	"sync"
 )
 
 var ErrTransportClosed = errors.New("protocol: transport closed")
@@ -18,10 +19,14 @@ type Transport interface {
 	Close() error
 }
 
-// ConnTransport is a JSON-lines implementation over a stream connection.
+// ConnTransport is a JSON-lines implementation over a stream connection. Send
+// is guarded by a mutex so the daemon can push frames from its subscriber pump
+// and its command handlers concurrently; Close may be called from any goroutine
+// (e.g. on context cancellation) to unblock a pending Recv.
 type ConnTransport struct {
 	conn net.Conn
 	r    *bufio.Reader
+	wmu  sync.Mutex
 	w    *bufio.Writer
 }
 
@@ -41,6 +46,8 @@ func (t *ConnTransport) Send(f Frame) error {
 	if err != nil {
 		return fmt.Errorf("protocol: encode frame: %w", err)
 	}
+	t.wmu.Lock()
+	defer t.wmu.Unlock()
 	if _, err := t.w.Write(append(b, '\n')); err != nil {
 		return err
 	}