fix: keep supervisor listen socket reusable across restarts

[richardmilles]

Bind with SO_REUSEPORT and add deploy/macos/overlap-restart-supervisor.sh so SIGTERM drain is not a hard outage.
Fixes #125

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Summary by cubic

Make supervisor restarts seamless by binding the public TCP listener with SO_REUSEPORT and adding a macOS overlap-restart helper. The new process can now bind before the old one drains, avoiding the drain-window outage.

• Bug Fixes
  • On Unix, bind the public listener with SO_REUSEPORT so a replacement supervisor can listen before SIGTERM closes the old process; non-Unix falls back to a normal Listen.
  • Add deploy/macos/overlap-restart-supervisor.sh to start a successor via launchd using the same ProgramArguments, then stop the old service, removing the multi-minute outage during restarts.

^{Written for commit ef4e3dd. Summary will update on new commits.}

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📥 Commits

Reviewing files that changed from the base of the PR and between 29c7ebb and ef4e3dd.

📒 Files selected for processing (5)

• cmd/subrouter/front.go
• cmd/subrouter/listener_reuseport_other.go
• cmd/subrouter/listener_reuseport_unix.go
• cmd/subrouter/supervisor.go
• deploy/macos/overlap-restart-supervisor.sh

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