internal/process: improve windows shutdown behaviour (#808)

Add Windows specific shutdown code paths so stopping of child processes
is more reliable:

- stopping llama-swap won't leave behind any child processes it created
- uses Job Objects in Windows so the whole llama-swap tree is closed by
the os
- add procCtx to baseRouter. It replaces shutdownCtx as a signal for
managing lifetime state.
- shutdownCtx is only used by the router to stop handling new requests
during shutdown
- improve debug logging to make it easier to trace source of issues

Fixes #804
Updates #807

internal/process/treecleanup_other.go

@@ -0,0 +1,7 @@
+//go:build !windows
+
+package process
+
+// SetupTreeCleanup is a no-op on non-Windows platforms, where upstream process
+// teardown is handled via process-group signalling (see runtime_unix.go).
+func SetupTreeCleanup() error { return nil }