add help docs and ?

docs/nodes/Field Arithmetic.md

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+# Field Arithmetic
+
+Apply a point-wise arithmetic operation to two DATA_FIELDs of the same resolution. Equivalent to gwy_data_field_sum_fields / subtract_fields / multiply_fields / divide_fields / min_of_fields / max_of_fields / hypot_of_fields.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field_a | DATA_FIELD | Yes | First operand |
+| field_b | DATA_FIELD | Yes | Second operand |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| result | DATA_FIELD | Result of the arithmetic operation (inherits metadata from field_a) |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| operation | dropdown | add | Element-wise operation: add, subtract, multiply, divide, min, max, or hypot (√(a²+b²)) |
+
+## Limitations
+
+- Both fields must have exactly the same pixel dimensions; mismatched sizes raise an error.
+- divide may produce NaN or Inf pixels where field_b is zero.