Make branch lines take their parent's color

A node's color now flows down the lines to its children, mind-map style:
Watering's teal colors the lines to Rain barrel and Drip line. It used to
color the line from the root down to Watering itself, which also left the
shared bus of a parent's branches in whichever child's color was drawn
last. Branches follow their parent by default, and stay dim when the
parent has no color.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

examples/garden.cum

@@ -43,9 +43,6 @@
             title: "Watering",
             pos: (22, 12),
             color: "teal",
-            branch: (
-                color: "node",
-            ),
         ),
         (
             id: 7,