use mutex for ipc ownership

[Shazbot]

• Start the IPC server automatically without requiring the Start_IPC argument.
• Use a per-session named mutex so only one AssetEditor instance hosts the fixed IPC pipe.
• Keep additional instances waiting efficiently until the current owner exits.
• Automatically take over IPC ownership when the owner closes or crashes.
• Use a dedicated background thread because Windows mutexes must be released by the acquiring thread.
• Improve cancellation, shutdown, pipe cleanup, and retry handling.