feat(sidecar): wire Snapshot handler with serialization lock

Synchronizes backend.snapshot() so concurrent runners observe a
serialized hierarchy+screenshot pair, eliminating the cross-fade race
where two parallel reads describe different frames.

sidecar/src/main/kotlin/dev/sanderling/sidecar/DriverService.kt

@@ -16,6 +16,7 @@ import dev.sanderling.driver.v1.Point
 import dev.sanderling.driver.v1.PressKeyRequest
 import dev.sanderling.driver.v1.RecentLogsRequest
 import dev.sanderling.driver.v1.Selector
+import dev.sanderling.driver.v1.SnapshotResponse
 import dev.sanderling.driver.v1.SwipeRequest
 import dev.sanderling.driver.v1.Text
 import io.grpc.stub.StreamObserver
@@ -27,6 +28,7 @@ class DriverService(
 ) : DriverGrpc.DriverImplBase() {
 
     private val launchedBundleId = AtomicReference<String?>(null)
+    private val snapshotLock = Any()
 
     override fun launch(request: LaunchRequest, responseObserver: StreamObserver<Empty>) {
         runRpc(responseObserver) {
@@ -116,6 +118,23 @@ class DriverService(
         }
     }
 
+    override fun snapshot(request: Empty, responseObserver: StreamObserver<SnapshotResponse>) {
+        runRpc(responseObserver) {
+            val sample = synchronized(snapshotLock) { backend.snapshot() }
+            val (png, width, height) = sample.screenshot
+            SnapshotResponse.newBuilder()
+                .setHierarchy(HierarchyJSON.newBuilder().setJson(sample.hierarchyJson).build())
+                .setScreenshot(
+                    Image.newBuilder()
+                        .setPng(ByteString.copyFrom(png))
+                        .setWidth(width)
+                        .setHeight(height)
+                        .build(),
+                )
+                .build()
+        }
+    }
+
     override fun waitForIdle(request: Duration, responseObserver: StreamObserver<Empty>) {
         runRpc(responseObserver) {
             backend.waitForIdle(request.millis)