Sidecar process-tree cleanup on Windows: no Job Object / tree kill, no parent-death watchdog

From the multi-agent Windows-robustness audit (adversarially verified findings).

Host side (src/sidecar/manager.rs ~216, ~300): killing a sidecar kills only the direct child. When the spawn fallback is dotnet run, the real sidecar is a grandchild and survives — still owning the named pipe, so the respawn collides with it. Roslyn BuildHost / MSBuild worker children are orphaned the same way. Windows needs a Job Object (or equivalent tree kill) so the whole tree dies with the child.

Sidecar side (sidecars/SharpLsp.Sidecar.Common/SidecarHost.cs ~124): if the host dies hard before the IPC connect, the sidecar waits on AcceptStreamAsync forever — no parent-process watchdog. Every hard host crash leaks a .NET process holding the pipe name.

Fix directions: assign the child to a Job Object with JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE on Windows; pass the host PID to the sidecar and have it exit when the parent exits (Process.GetProcessById + Exited, or WaitForSingleObject on the parent handle).

Related: #150–#155 (sidecar lifecycle hardening). Found while fixing #110 follow-ups.

Read src/sidecar/manager.rs around lines 216 and 300, then inspect sidecars/SharpLsp.Sidecar.Common/SidecarHost.cs around line 124. Trace the Windows spawn and cleanup paths first; done means the host cleanup terminates the complete sidecar tree and the sidecar exits when its parent dies, including before IPC connection.