fix: ttyd dies immediately — add start_new_session=True to subprocess spawn

Without start_new_session=True, ttyd inherits the parent's process group
and gets cleaned up when the asyncio transport is garbage collected after
the HTTP handler completes. start_new_session=True detaches ttyd into
its own process group so it survives independently of the parent.

muxplex/ttyd.py

@@ -213,6 +213,7 @@ async def spawn_ttyd(session_name: str) -> asyncio.subprocess.Process:
         session_name,
         stdout=asyncio.subprocess.DEVNULL,
         stderr=asyncio.subprocess.DEVNULL,
+        start_new_session=True,  # detach from parent process group so ttyd survives independently
     )
 
     # Write PID file (create parent dirs if needed)