fix: address P0-2b review issues - build tag and actionType guard

1. Add //go:build !cgo to control_host_test.go so the stub-testing
   tests only run in non-CGO builds, matching the stub.go build
   constraint. Clean up dead nil check in SignatureCompiles test.

2. Replace two independent if-statements for actionType mapping in
   ControlHost with a map lookup + explicit guard, consistent with
   ControlQueue's ActionMap pattern. Prevents silent zero-value
   actionType if SupportedHostActions expands later.

src/frontend/apiserver/backend/control_host_test.go

@@ -1,3 +1,5 @@
+//go:build !cgo
+
 package backend
 
 import (
@@ -18,8 +20,5 @@ func TestControlHost_StubReturnsError(t *testing.T) {
 // signature matches: func ControlHost(hostname string, action int) error.
 // If the signature changes this test will not compile.
 func TestControlHost_SignatureCompiles(t *testing.T) {
-	var fn func(string, int) error = ControlHost
-	if fn == nil {
-		t.Fatal("ControlHost must not be nil")
-	}
+	var _ func(string, int) error = ControlHost
 }