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/ui/selection.js

@@ -82,7 +82,7 @@ function setBrush(d) {
 // The block variant a piece is placed with: wall material, or direction (a filter's
 // materials and mode travel separately, as GAME.filterCfg)
 function pieceV(P) {
-  if (P.code === BP_WALL) return GAME.wallMat;
+  if (isWall(P.code)) return GAME.wallMat;
   if (P.filter) return (GAME.pieceDir[P.id] || 0) << 7;
   return GAME.pieceDir[P.id] || 0;
 }
@@ -91,6 +91,17 @@ function filterBlockAt(x, y) { if (x < 0 || y < 0 || x >= W || y >= H) return -1
 function chipCost(cost) { const ok = canAfford(cost, 1); return `<span class="chip ${ok ? 'ok' : 'bad'}">${esc(costText(cost, 1))}</span>`; }
 function pieceCostNow(P, m = GAME.wallMat) {
   if (P.kind === 'mold') return wallCost(m, MOLDS[P.mold].walls);
-  if (P.kind === 'block' && P.code === BP_WALL) return wallCost(m, BS * BS);
+  if (P.kind === 'block' && isWall(P.code)) return pieceCost(P.code, m);
   return P.cost;
 }
+
+function pieceCode(P, end = aimBlock()) {
+  if (!isSlope(P.code)) return P.code;
+  const a = GAME.drag || end;
+  return slopeCode(end.bx - a.bx, end.by - a.by, GAME.pieceDir.slope || 0);
+}
+function turnSlope() {
+  const n = GAME.pieceDir.slope || 0;
+  GAME.pieceDir.slope = GAME.drag ? n ^ 2 : (n + 1) % 4;
+  hudDirty = true;
+}