fix(layers): use KiCad 9 API for add_layer

board.GetLayerStack() was removed in KiCad 9. Call SetLayerName and
SetLayerType directly on the board instead, and grow the copper layer
count via SetCopperLayerCount when adding inner layers. Without this,
add_layer raises AttributeError on any KiCad 9 installation.

python/commands/board/layers.py

@@ -39,9 +39,6 @@ class BoardLayerCommands:
                     "errorDetails": "name, type, and position are required",
                 }
 
-            # Get layer stack
-            layer_stack = self.board.GetLayerStack()
-
             # Determine layer ID based on position and number
             layer_id = None
             if position == "inner":
@@ -64,12 +61,16 @@ class BoardLayerCommands:
                     "errorDetails": "position must be 'top', 'bottom', or 'inner'",
                 }
 
-            # Set layer properties
-            layer_stack.SetLayerName(layer_id, name)
-            layer_stack.SetLayerType(layer_id, self._get_layer_type(layer_type))
+            # Enable inner copper layers by increasing copper layer count (KiCAD 9.0 API)
+            if position == "inner":
+                current_count = self.board.GetCopperLayerCount()
+                needed_count = 2 + number  # F.Cu + B.Cu + inner layers
+                if needed_count > current_count:
+                    self.board.SetCopperLayerCount(needed_count)
 
-            # Enable the layer
-            self.board.SetLayerEnabled(layer_id, True)
+            # Set layer properties directly on board (GetLayerStack removed in KiCAD 9.0)
+            self.board.SetLayerName(layer_id, name)
+            self.board.SetLayerType(layer_id, self._get_layer_type(layer_type))
 
             return {
                 "success": True,