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>

src/sim/undo.js

@@ -61,7 +61,7 @@ function ghostTick0() {
       ghostBlocks.delete(b); ghostBlocked.delete(b); continue;
     }
     if (!blockInReach(bx, by)) continue;
-    const err = canPlaceBlock(bx, by);
+    const err = canPlaceBlock(bx, by, code);
     if (err) { if (err.startsWith('Needs')) ghostBlocked.add(b); continue; }
     if (!canAfford(pieceCost(code, v), 1)) { ghostMissing++; continue; }
     placeBlock(code, v, bx, by, false, e);