add help docs and ?

docs/nodes/Save.md

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+# Save
+
+Save a single value to disk. Supports fields, images, lines, tables, scalars, and 3D meshes. Does not auto-run; click Save to write.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| value | DATA_FIELD, IMAGE, ANNOTATION_SOURCE, LINE, RECORD_TABLE, DATA_TABLE, MESH_MODEL, or FLOAT | Yes | Value to save |
+| directory | DIRECTORY | No | Optional directory path from a Folder node; overrides the directory_path widget |
+
+## Outputs
+
+None.
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| filename | STRING | "" | Output filename |
+| directory_path | FOLDER_PICKER | "" | Output directory; hidden when directory input is connected |
+| format | STRING (context-dependent) | TIFF | Output format; available choices depend on the connected input type |
+| plot_title | STRING | "" | Optional title for line plots saved as PNG or TIFF |
+
+## Limitations
+
+- Available formats per input type: DATA_FIELD → TIFF, PNG, NPZ; IMAGE → PNG, TIFF, NPZ; LINE → PNG, TIFF, CSV, NPZ, JSON; RECORD_TABLE/DATA_TABLE → CSV, JSON; FLOAT → TXT, JSON; MESH_MODEL → OBJ, STL.
+- divide-by-zero or NaN values in fields are preserved in TIFF/NPZ but may not render correctly in PNG.