add help docs and ?

docs/nodes/Mask Invert.md

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+# Mask Invert
+
+Invert a binary mask — swap masked and unmasked regions.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| mask | IMAGE | Yes | Binary mask to invert |
+| field | DATA_FIELD | No | Optional field for preview background display |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| mask | IMAGE | Inverted binary mask |
+
+## Controls
+
+None.
+
+## Limitations
+
+- None.