refactor nodes into standalone file

backend/nodes/color_map.py

@@ -0,0 +1,48 @@
+from __future__ import annotations
+import json
+from backend.node_registry import register_node
+from backend.data_types import COLORMAPS, DEFAULT_CUSTOM_COLORMAP_STOPS, normalize_colormap_spec
+
+
+@register_node(display_name="Color Map")
+class ColorMap:
+    @classmethod
+    def INPUT_TYPES(cls):
+        return {
+            "required": {
+                "mode": (["preset", "custom"], {"default": "preset"}),
+                "preset": (list(COLORMAPS), {
+                    "default": "viridis",
+                    "show_when_widget_value": {"mode": ["preset"]},
+                }),
+                "stops": ("STRING", {
+                    "default": json.dumps(list(DEFAULT_CUSTOM_COLORMAP_STOPS)),
+                    "colormap_stops": True,
+                    "show_when_widget_value": {"mode": ["custom"]},
+                }),
+            }
+        }
+
+    RETURN_TYPES = ("COLORMAP",)
+    RETURN_NAMES = ("colormap",)
+    FUNCTION = "build"
+
+    DESCRIPTION = (
+        "Build a reusable colormap. Choose a preset, or create a custom gradient with min/max colours "
+        "and any number of intermediate stops."
+    )
+
+    def build(self, mode: str, preset: str, stops: str | None = None, stops_json: str | None = None) -> tuple:
+        if mode == "preset":
+            return ({"mode": "preset", "preset": normalize_colormap_spec(preset)},)
+
+        try:
+            raw_stops = stops if stops is not None else stops_json
+            stops_data = json.loads(raw_stops or "[]")
+        except json.JSONDecodeError as exc:
+            raise ValueError("Custom colormap stops must be valid JSON.") from exc
+
+        spec = normalize_colormap_spec({"mode": "custom", "stops": stops_data}, fallback=None)
+        if not (isinstance(spec, dict) and spec.get("mode") == "custom"):
+            raise ValueError("Custom colormap must include at least min and max colours.")
+        return (spec,)