feat(driver): native DoubleTap RPC for a tight inter-tap gap

Composing two Tap round trips from the Go client spread the taps by
hundreds of milliseconds on iOS, wide enough for the app to navigate
between them, so double-submission races could never reproduce. The
sidecar now lands both taps back-to-back next to the device transport.

proto/driverpb/driver.proto

@@ -11,6 +11,11 @@ service Driver {
   rpc Terminate(Empty) returns (Empty);
 
   rpc Tap(Point) returns (Empty);
+  // DoubleTap lands two taps as close together as the platform allows.
+  // Composing two Tap calls from the client spreads them by hundreds of
+  // milliseconds on iOS, wide enough for navigation to interleave, which
+  // defeats double-submission testing.
+  rpc DoubleTap(Point) returns (Empty);
   rpc TapSelector(Selector) returns (Empty);
   rpc InputText(Text) returns (Empty);
   // EraseText deletes characters from the focused field so InputText can