Sloped walls with a flat 45° face, and diagonal wall lines (t36)

- New Sloped wall piece: a half-block triangle in any wall material, at half a wall's cost.
  Dragging walls or slopes at about 45° lays a diagonal line; the drag picks the slope's
  orientation and T flips or turns it.
- Each slope fills one cell past its corner, so a diagonal run is sealed against grains and water.
- New test/t_slopes.js: a slope funnel passes 400/400 sand (square steps 379/400).

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

docs/DESIGN.md

@@ -364,6 +364,15 @@ above, including:
   tick, nearest first (a Huge world is fully started after 17 s), and flying toward areas still
   waiting starts them. Nothing is skipped: the same settling happens, spread out.
 
+- **Sloped walls and diagonal wall lines (t36, 2026-10-25).** The owner wanted funnel walls that
+  material slides down instead of catching on block steps. A Sloped wall is a half-block triangle
+  with a flat 45° face, in four orientations, built of any wall material at half a wall block's
+  cost. Dragging walls or slopes at about 45° lays a diagonal line; for slopes the drag direction
+  picks the orientation (solid side below) and T flips it, or turns a single piece. Each slope
+  also fills one cell past its corner so a diagonal run is sealed (grains move diagonally and
+  would otherwise slip through where two pieces meet). Square walls placed diagonally still
+  touch only at corners and leak there, like any square blocks.
+
 ## Ideas not built yet
 
 - **Concrete**, as a stronger wall material from rubble.