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]>

DESIGN.md

@@ -77,7 +77,9 @@ lines.
 - Connections can set arrowheads (off, end, start, both), a line (solid,
   dashed, dotted, thick, double), a color (a palette color or the node's own)
   and a label, or use a named style. Branches start solid without arrowheads,
-  cross-links as dashed arrows.
+  in their parent's color, so a node's color flows down the lines to its
+  children (dim when the parent has no color). Cross-links start as dim
+  dashed arrows.
 
 **Keys**
 - Medium vim: counts, operators with node targets, registers (with `"+` for the