add help docs and ?

docs/nodes/Curvature.md

@@ -0,0 +1,31 @@
+# Curvature
+
+Fit a quadratic surface and report the overall principal curvature radii and directions, matching Gwyddion's curvature feature. The output annotation marks the principal cross-sections and the node also returns the two corresponding height profiles.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Input topographic field |
+| mask | IMAGE | No | Binary mask for including or excluding regions from the quadratic fit |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| output | ANNOTATION_SOURCE | Field with principal-axis lines overlaid |
+| measurements | RECORD_TABLE | Curvature radii, principal-axis directions, center position and value |
+| profile_a | LINE | Height profile along principal axis 1 |
+| profile_b | LINE | Height profile along principal axis 2 |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| masking | dropdown | ignore | How to use the mask: ignore (fit all pixels), include (fit only masked pixels), or exclude (fit unmasked pixels) |
+
+## Limitations
+
+- Requires at least six usable pixels for the quadratic fit; fewer pixels produce no output.
+- The fit assumes a globally smooth quadratic surface; locally rough or step-like surfaces give unreliable results.
+- Requires compatible XY and Z physical units (e.g. both in metres).