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]>

src/model/look.rs

@@ -49,8 +49,9 @@ pub enum Arrows {
 pub enum LineColor {
     /// A color from the theme's palette.
     Named(ColorName),
-    /// The color of the node the line comes from (for a branch: the child),
-    /// so each branch takes its node's color, mind-map style.
+    /// The color of the node the line comes from: a branch takes its
+    /// parent's (so a node's color flows down the lines to its children,
+    /// mind-map style), a cross-link its start's.
     FollowNode,
 }