add help docs and ?

docs/nodes/Grain Analysis.md

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+# Grain Analysis
+
+Label connected grain regions in a binary mask and compute per-grain statistics: area, equivalent diameter, mean/max height, bounding box. Equivalent to Gwyddion's grain statistics tools.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Height field providing z values for per-grain statistics |
+| mask | IMAGE | Yes | Binary mask defining grain regions (white = grain) |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| grain_stats | DATA_TABLE | Per-grain table with area, equivalent diameter, mean/max height, bounding box, and centroid |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| min_size | INT | 10 | Minimum grain area in pixels; smaller connected regions are ignored (1–100000) |
+
+## Limitations
+
+- Grain detection uses 2D connected-component labeling on the binary mask; the field and mask must have the same pixel dimensions.
+- Physical area and diameter values require the field to carry valid physical calibration (xreal, yreal, si_unit_xy).