Add restartable dev server lifecycle

[chaptersix]

Summary

• allow callers to supply and preserve a complete dev-server port allocation
• add a serialized Server.Restart lifecycle that reuses the built binary, configuration, work directory, ports, and output writer
• keep Stop final and idempotent while handling restart failures and cancellation cleanly

Test plan

• go test ./devserver
• go test -race ./devserver
• git diff --check