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>

src/sim/pieces.js

@@ -32,6 +32,8 @@ function slopeCode(dx, dy, orientation) {
   return dx * dy > 0 ? (orientation >= 2 ? BP_SLOPE_TR : BP_SLOPE_BL) : (orientation >= 2 ? BP_SLOPE_TL : BP_SLOPE_BR);
 }
 function isPipeCode(c) { return c === BP_PIPE || c === BP_PUMP || c === BP_OUTLET; }
+// pieces that carry something along a run: a Shift-drag still draws a line, never a filled rectangle
+function lineOnly(c) { return isBelt(c) || isFilter(c) || c === BP_SCREW || c === BP_SHAFT || isPipeCode(c); }
 // Filters: each filter block has a set of materials and a mode. In drop mode (inv 0)
 // the chosen materials fall through and everything else rides along on top; in carry
 // mode (inv 1) the chosen ones ride along and everything else falls through. The block