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/tools.js

@@ -34,7 +34,7 @@ function toolDig(cx, cy, r) {
       const b = blockP[((y / BS) | 0) * BW + ((x / BS) | 0)];
       setCell(i, to, temp[i], 0);
       discover(to, x, y, m, 'dig');
-      if (b === BP_WALL && !blockIntegrity(((y / BS) | 0) * BW + ((x / BS) | 0))) blockP[((y / BS) | 0) * BW + ((x / BS) | 0)] = 0;
+      if (isWall(b) && !blockIntegrity(((y / BS) | 0) * BW + ((x / BS) | 0))) removeBlock((x / BS) | 0, (y / BS) | 0);
       dug++;
       if ((dug & 3) === 0) emit({ t: 'debris', x, y, m });
     } else digp[i] = p;