refactor nodes into standalone file

backend/nodes/mask_invert.py

@@ -0,0 +1,41 @@
+from __future__ import annotations
+import numpy as np
+from backend.node_registry import register_node
+from backend.data_types import DataField, encode_preview
+from backend.nodes.helpers import _mask_overlay
+
+
+@register_node(display_name="Mask Invert")
+class MaskInvert:
+    _CUSTOM_PREVIEW = True
+
+    @classmethod
+    def INPUT_TYPES(cls):
+        return {
+            "required": {
+                "mask": ("IMAGE",),
+            },
+            "optional": {
+                "field": ("DATA_FIELD",),
+            }
+        }
+
+    RETURN_TYPES = ("IMAGE",)
+    RETURN_NAMES = ("mask",)
+    FUNCTION = "process"
+
+    DESCRIPTION = "Invert a binary mask — swap masked and unmasked regions."
+
+    _broadcast_fn = None
+    _current_node_id: str = ""
+
+    def process(self, mask: np.ndarray, field: DataField | None = None) -> tuple:
+        out = np.where(mask > 127, np.uint8(0), np.uint8(255))
+
+        if field is not None and MaskInvert._broadcast_fn is not None:
+            overlay = _mask_overlay(field, out)
+            MaskInvert._broadcast_fn(
+                MaskInvert._current_node_id, encode_preview(overlay),
+            )
+
+        return (out,)