Wrap tool.System.get_decoration_data with TokenCache lookup

System decoration draws on every viewport refresh — the
``_build_decoration_data`` body walks every distribution element,
resolves connected ports, builds the vert/edge arrays for the GPU
batch. A bare call per frame burns time on an unchanged scene.

Add a single-entry cache keyed on ``(decorator_cache_token,
id(decorated_elements_set))``. Reads short-circuit when neither
component moved:

* ``decorator_cache_token`` from ``bim.decorator_cache`` invalidates
  on depsgraph / undo / redo / load via the bump handler.
* ``id(decorated_elements_set)`` invalidates when
  ``SystemDecorationData.load()`` reassigns the set (e.g. when the
  user changes the set of decorated systems via the panel).

The handler that bumps the token is installed in the next commit
(bim/handler.py decompose). Until then the token stays at 0, so
the cache only hits when ``id()`` also matches — degraded behaviour
during the bisect window but not incorrect.

Generated with the assistance of an AI coding tool.

src/bonsai/bonsai/tool/system.py

@@ -299,15 +299,29 @@ class System(bonsai.core.tool.System):
         system_props = cls.get_system_props()
         return tool.Ifc.get_entity_by_id(system_props.active_system_id)
 
+    # Decoration-data cache, keyed on (decorator_cache_token, id(decorated_elements_set)).
+    _decoration_data_cache_key: tuple | None = None
+    _decoration_data_cache: dict[str, Any] | None = None
+
     @classmethod
     def get_decoration_data(cls) -> dict[str, Any]:
+        from bonsai.bim.decorator_cache import get_decorator_cache_token
         from bonsai.bim.module.system.data import ObjectSystemData, SystemDecorationData
 
         if not ObjectSystemData.is_loaded:
             ObjectSystemData.load()
         if not SystemDecorationData.is_loaded:
             SystemDecorationData.load()
-        return cls._build_decoration_data()
+
+        token = get_decorator_cache_token()
+        key = (token, id(SystemDecorationData.data["decorated_elements"]))
+        if key == cls._decoration_data_cache_key and cls._decoration_data_cache is not None:
+            return cls._decoration_data_cache
+
+        result = cls._build_decoration_data()
+        cls._decoration_data_cache_key = key
+        cls._decoration_data_cache = result
+        return result
 
     @classmethod
     def _build_decoration_data(cls) -> dict[str, Any]: