add help docs and ?

docs/nodes/Cross-Correlate.md

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+# Cross-Correlate
+
+Compute 2D cross-correlation between two fields. The correlation peak indicates the offset where the two fields best match. Useful for drift measurement and feature alignment. Equivalent to Gwyddion crosscor.c.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field_a | DATA_FIELD | Yes | First input field |
+| field_b | DATA_FIELD | Yes | Second input field |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| correlation | DATA_FIELD | Cross-correlation map |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| mode | dropdown | same | Output size: full (Na+Nb−1), same (same as field_a), or valid (overlapping region only) |
+| normalize | BOOLEAN | True | Normalize the result to [−1, 1] by dividing by the product of RMS values |
+
+## Limitations
+
+- Both fields must be of compatible numpy array types; very different sizes in full mode produce large outputs.
+- Mean is subtracted before correlation; absolute offset information is lost.