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

go.mod

@@ -15,6 +15,7 @@ require (
 	github.com/tidwall/gjson v1.18.0
 	github.com/tidwall/sjson v1.2.5
 	golang.org/x/sync v0.20.0
+	golang.org/x/sys v0.41.0
 	gopkg.in/yaml.v3 v3.0.1
 )
 
@@ -70,7 +71,6 @@ require (
 	golang.org/x/arch v0.8.0 // indirect
 	golang.org/x/crypto v0.45.0 // indirect
 	golang.org/x/net v0.47.0 // indirect
-	golang.org/x/sys v0.41.0 // indirect
 	golang.org/x/text v0.31.0 // indirect
 	google.golang.org/protobuf v1.34.1 // indirect
 )