add help docs and ?

docs/nodes/Grain Distance Transform.md

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+# Grain Distance Transform
+
+Compute the mask distance transform using Gwyddion-style interior, exterior, or signed output. Supports Euclidean, city-block, chessboard, and octagonal distance variants, with optional image-boundary handling matching mask_edt.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Field providing physical calibration for the output |
+| mask | IMAGE | Yes | Binary mask to compute distances for |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| distance | DATA_FIELD | Distance transform field in physical units |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| distance_type | dropdown | euclidean | Distance metric: euclidean, cityblock, chess (Chebyshev), octagonal48, octagonal84, or octagonal |
+| output_type | dropdown | interior | Output mode: interior (distances inside grains), exterior (distances outside), or signed (positive inside, negative outside) |
+| from_border | BOOLEAN | True | When enabled, image borders are treated as mask boundaries |
+
+## Limitations
+
+- Output distances are in physical xy units derived from the field calibration.
+- The mask must have the same pixel dimensions as the field.