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/agent-log.md

@@ -4,6 +4,17 @@ What agents have done, newest first. Add an entry for every piece of work: the d
 note or request it was for, what changed, the files touched, the tests run, and anything left
 open or not verified. Keep each entry short; the git history has the details.
 
+## 2026-10-27: Shift-drag draws a line for transport pieces (Claude Code)
+
+- Owner request: Shift shouldn't fill a rectangle for belts, conveyors and similar. New
+  `lineOnly(code)` in `src/sim/pieces.js` (belts, filters, screw conveyors, shafts, pipe pieces);
+  `primaryRelease` (`src/ui/actions.js`) and the drag preview (`src/ui/previews.js`) ignore Shift
+  for them, and `pieceCode` (`src/ui/selection.js`) no longer flattens a diagonal belt drag on
+  Shift. Build-menu "How to place" and the notebook say so; the notebook's stale "cleats up or
+  down" line now says uphill only.
+- Tests: play.py checks a Shift-drag with a belt lays a 3-piece sloped line. Full
+  `run_all --browser` passes.
+
 ## 2026-10-27: t61 sticky materials slide freely on built surfaces (Claude Code)
 
 - The owner's video showed clay lumps creeping down a sloped wall; removing a piece was