Diagonal screw tubes, laid by a 45° drag (t35)

- Tubes run in eight directions: a drag at about 45° lays a diagonal run, R cycles all eight.
- Diagonal runs carry grains at the straight rate (120 grains/s), join straight tubes and belts,
  keep grains in at the corners, pass water, and carry bars level.
- New part ids 11-14 (saved ids are never renumbered); tube checks use an IS_TUBE table.
- New test/t_diag_tubes.js (85 checks).

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

docs/DESIGN.md

@@ -373,6 +373,11 @@ above, including:
   would otherwise slip through where two pieces meet). Square walls placed diagonally still
   touch only at corners and leak there, like any square blocks.
 
+- **Diagonal screw tubes (t35, 2026-10-25).** Tubes run in eight directions. A drag at about 45°
+  lays a diagonal run pointing along the drag, and R cycles all eight for a single tube. Diagonal
+  runs carry grains at the same rate as straight ones, join straight tubes and belts, and carry
+  bars level, a block every eight ticks. They use new part ids (11-14) because part ids are saved.
+
 ## Ideas not built yet
 
 - **Concrete**, as a stronger wall material from rubble.