fix(dev): give serve the full shutdown grace before killing it

- dev.ShutdownGrace (internal, 15 seconds, longer than serve's 10-second
  drain) is the grace child.stop waits after SIGTERM, instead of 2 seconds
- summer's delegates use the same constant (appShutdownGrace)
- scaffolding docs say summer dev waits for serve's shutdown report (WR-03)

cmd/summer/runtime.go

@@ -11,9 +11,9 @@ import (
 	"path/filepath"
 	"strings"
 	"syscall"
-	"time"
 
 	"git.golem15.com/golem15/summercms/internal/build"
+	"git.golem15.com/golem15/summercms/internal/dev"
 	"git.golem15.com/golem15/summercms/modules/bonfire"
 )
 
@@ -181,9 +181,9 @@ func runAppCmd(cmd *exec.Cmd, argv []string) error {
 }
 
 // appShutdownGrace is how long a cancelled app binary may take to exit after
-// SIGTERM before it is killed. It is longer than the 10-second shutdown
-// timeout of serve and the workers, so they finish a graceful shutdown.
-const appShutdownGrace = 15 * time.Second
+// SIGTERM before it is killed: the grace summer dev gives serve as well,
+// longer than the 10-second shutdown timeout of serve and the workers.
+const appShutdownGrace = dev.ShutdownGrace
 
 // newAppCmd builds the command that runs the app binary bin with argv in
 // dir, writing to stdout and stderr (see appStreams). The child inherits