add help docs and ?

docs/nodes/Kuwahara Filter.md

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+# Kuwahara Filter
+
+Edge-preserving smoothing using Kuwahara's minimum-variance quadrant method. Unlike Gaussian blur, sharp boundaries are preserved. Equivalent to Gwyddion's Kuwahara filter.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input field to filter |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| filtered | DATA_FIELD | Smoothed field with preserved edges |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| iterations | INT | 1 | Number of times the 5×5 Kuwahara pass is applied (1–20) |
+
+## Limitations
+
+- The kernel is fixed at 5×5 pixels; coarser smoothing requires more iterations.
+- Multiple iterations increase processing time proportionally; 20 iterations on a large field may be slow.