feat(proto): add TapSelector RPC and Selector message

Driver interface in Go already exposes TapSelector for selector-based
taps; mirror that in the proto so maestro can resolve selectors
sidecar-side rather than forcing Go to walk the hierarchy first.

proto/driverpb/driver.proto

@@ -11,6 +11,7 @@ service Driver {
   rpc Terminate(Empty) returns (Empty);
 
   rpc Tap(Point) returns (Empty);
+  rpc TapSelector(Selector) returns (Empty);
   rpc InputText(Text) returns (Empty);
 
   rpc Screenshot(Empty) returns (Image);
@@ -36,6 +37,10 @@ message Text {
   string value = 1;
 }
 
+message Selector {
+  string value = 1;
+}
+
 message Image {
   bytes png = 1;
   int32 width = 2;