add help docs and ?

docs/nodes/Local Contrast.md

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+# Local Contrast
+
+Expand the local dynamic range at each pixel to reveal fine surface features that are hidden by global contrast range. Equivalent to Gwyddion local_contrast.c.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input field |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| result | DATA_FIELD | Field with enhanced local contrast |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| kernel_size | INT | 10 | Size of the local neighbourhood window in pixels (2–100) |
+| weight | FLOAT | 0.5 | Blend weight between original and full-contrast output (0 = original, 1 = full local contrast; 0–1) |
+
+## Limitations
+
+- Large kernel sizes are slow; values above ~50 pixels may be noticeably slow on large fields.
+- The enhancement is purely a display-contrast operation; it changes the underlying data values.