feat(ios): derive scrollable from the snapshot's tree depth

the companion now emits each node's depth, so the hierarchy mapper can find the containers that clip content reaching past their own frame and mark them scrollable:true, the same fact android reads off uiautomator and the web driver derives from overflow. a dump without depth makes every element a root and roots are never marked, so the legacy bridge reports no scroll rather than a guessed one.

companion/Sources/Snapshot.swift

@@ -33,27 +33,28 @@ enum Snapshot {
         let application = XCUIApplication(bundleIdentifier: bundleIdentifier)
         let root = try application.snapshot()
         var result: [[String: Any]] = []
-        walk(root, into: &result)
+        walk(root, depth: 0, into: &result)
         return result
     }
 
-    private static func walk(_ node: XCUIElementSnapshot, into result: inout [[String: Any]]) {
+    private static func walk(_ node: XCUIElementSnapshot, depth: Int, into result: inout [[String: Any]]) {
         // The keyboard subtree is pruned: the legacy accessibility bridge
         // never exposed it, its key frames are unreliable as tap targets, and
         // its shift-state churn destabilizes settle hashing.
         if node.elementType == .keyboard {
             return
         }
-        result.append(serialize(node))
+        result.append(serialize(node, depth: depth))
         for child in node.children {
-            walk(child, into: &result)
+            walk(child, depth: depth + 1, into: &result)
         }
     }
 
-    private static func serialize(_ node: XCUIElementSnapshot) -> [String: Any] {
+    private static func serialize(_ node: XCUIElementSnapshot, depth: Int) -> [String: Any] {
         let frame = node.frame
         return [
             "type": elementTypeName(node.elementType),
+            "depth": depth,
             "frame": [
                 "x": Double(frame.origin.x),
                 "y": Double(frame.origin.y),