TestServer had no Drop and std's Child does not kill on drop, so any test
that panicked, returned early or skipped shutdown() left its
signaltty-server running forever (dozens piled up across worktrees). A
startup readiness timeout leaked the child the same way.

Drop now kills, waits and removes the per-test socket/state dir; the
server is owned before the readiness wait so a startup panic reaps it.
shutdown() keeps the graceful request and leaves the reaping to Drop.

Co-authored-by: Lucenx9 <185146821+Lucenx9@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>