feat(ksuid): add Equal and IsZero for ApplyInterfacePtr

StringKSUID and BinaryKSUID now implement ApplyInterface with nil-nil
equality semantics, enabling ApplyInterfacePtr in patch logic. Adds nil
sentinel vars and tests.

Co-authored-by: Cursor <cursoragent@cursor.com>

ksuid_gql.go

@@ -28,3 +28,10 @@ const (
 	ksuidBinaryLength = 20
 	ksuidStringLength = 27
 )
+
+func equalKSUID(lh, rh ksuid.KSUID) bool {
+	if lh.IsNil() && rh.IsNil() {
+		return true
+	}
+	return lh.String() == rh.String()
+}