feat(sidecar): add snapshot default on DriverBackend

Default impl calls hierarchy() then screenshot(). The service layer wraps
the call in a mutex so concurrent runners observe a serialized pair.

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

@@ -14,8 +14,19 @@ interface DriverBackend {
     fun waitForIdle(durationMillis: Long)
     fun healthy(): Boolean
     fun metrics(bundleId: String): MetricsSample
+
+    // snapshot captures hierarchy then screenshot back-to-back. The service
+    // layer holds a mutex around the call so concurrent callers observe a
+    // serialized pair from the same on-device frame. Backends may override
+    // to fuse the two reads more tightly when their native API allows.
+    fun snapshot(): SnapshotSample = SnapshotSample(hierarchy(), screenshot())
 }
 
+data class SnapshotSample(
+    val hierarchyJson: String,
+    val screenshot: Triple<ByteArray, Int, Int>,
+)
+
 // STABILITY_POLL_INTERVAL_MILLIS is set wide enough that UiAutomation /
 // Maestro's contentDescriptor doesn't get hammered: tighter intervals were
 // observed to back the sidecar gRPC stream up under fuzz load to the point