Shift-drag draws a line for belts and other transport pieces

Belts, filters, screw conveyors, shafts and pipe pieces never fill a rectangle;
a diagonal belt drag stays sloped with Shift held.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/DESIGN.md

@@ -477,6 +477,10 @@ above, including:
   chose to make stickiness grain against grain only: on anything built, every material slides
   freely, so slopes are useful for every material, while heaps keep their shape. Real chutes clog
   with sticky material and need cleaning, but that is micromanagement the game doesn't need.
+- **Belts and other transport pieces never fill a rectangle (2026-10-27).** The owner said Shift
+  doesn't need to fill a rectangle for belts, conveyors and the like. Belts, filters, screw
+  conveyors, shafts, pipes, pumps and outlets (`lineOnly()`) always draw a line, Shift or not, and
+  a diagonal belt drag stays sloped. Walls, grates and gates still Shift-fill.
 
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