Joins follow the travel direction: the next piece goes right after the end (t67 phase 2b)

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

docs/DESIGN.md

@@ -486,16 +486,20 @@ above, including:
   on warns first that the save will be marked, and the mark stays with the world; any future
   achievements won't unlock in a marked world. Besides money, items and upgrades it may have
   game speed and other things just for fun.
-- **One join rule for every transport piece (2026-10-27, replaces the t62 same-row rule).** The
-  owner wants all pieces to place and join the same way, so players know what to expect when
-  building. At a diagonal piece's end, call E the block diagonally past its last block (the
-  block its end points into). Another diagonal of the same slope continues in E. A horizontal
-  piece (flat belt, flat filter, horizontal screw) joins on the same row at the high end, and
-  in E, one block lower, at the low end: the rule sloped belts already had. A vertical screw
-  joins in E at either end. Straight pieces join the next block along their axis. Any other
-  placement is two separate ends and nothing passes between them. The owner found the old
-  screw rule, which needed the horizontal placed over the diagonal's last block at the low
-  end, inconsistent with every other piece.
+- **One join rule for every transport piece: the next piece goes right after the end
+  (2026-10-27, replaces the t62 same-row rule).** The owner wants all pieces to place and join
+  the same way, so players know what to expect when building. Joins follow the direction the
+  material travels: a piece's outlet points at the block right after its end, which is the
+  next block along a straight piece, or the block diagonally past a diagonal's end. The next
+  piece joins when it starts in that block and carries the same general way. So a horizontal
+  then a diagonal (up or down) starts the diagonal in the next block on the same row; a
+  diagonal then a horizontal or vertical starts it diagonally past the diagonal's end; a
+  vertical going up then a diagonal starts the diagonal directly above. Any other placement is
+  two separate ends that pass nothing. Belts carry grains on top, so they bend the rule where
+  grains can't follow it: an uphill belt's top feeds a flat belt on the same row, and a flat
+  belt one block below an uphill belt's low end also feeds it. The owner first approved a rule
+  based only on shape, then found in testing that horizontal → uphill and vertical → diagonal
+  didn't join the way the other pieces did.
 
 ## Ideas not built yet