add help docs and ?

docs/nodes/Cursors.md

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+# Cursors
+
+Place two draggable cursors on a line plot or 2D field to measure positions and deltas. On lines it reports x/y positions and dx/dy; on fields it reports x/y/z at both markers plus dx/dy/dz.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| line | LINE or DATA_FIELD | Yes | Input line profile or 2D field |
+| coord_pair | COORDPAIR | No | Locks both cursor positions from an external coordinate pair |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| measurement | RECORD_TABLE | Positions, deltas, and lengths at the two cursor locations |
+| coord_pair | COORDPAIR | Current cursor positions (passthrough for chaining) |
+
+## Controls
+
+None (cursor positions are set by dragging in the preview panel).
+
+## Limitations
+
+- When a COORDPAIR is connected, the cursor positions are locked and cannot be dragged interactively.
+- On 2D fields, z values are sampled with bilinear (order 1) interpolation.