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/notebook.js

@@ -43,8 +43,8 @@ function renderNotebook() {
     let h = '<p class="note">Everything you build snaps to a grid of 4×4-grain blocks. Pieces only move, hold, blow or sense material; every change to the material itself comes from heat. Select pieces with the Build tool (3). Hover a tile for its cost, full description and placement help. Your bot flies straight through anything you build.</p><div class="cards">';
     for (const P of PIECES) {
       if (P.hidden) continue;
-      const cost = P.kind === 'mold' ? `${MOLDS[P.mold].walls} grains of wall material (R cycles, F opens the menu)` : P.code === BP_WALL ? '16 grains of rubble, lumps or chunks, or 2 bars' : costText(P.cost, 1);
-      const afford = P.kind === 'mold' || P.code === BP_WALL ? true : canAfford(P.cost, 1);
+      const cost = P.kind === 'mold' ? `${MOLDS[P.mold].walls} grains of wall material (R cycles, F opens the menu)` : isWall(P.code) ? (isSlope(P.code) ? '8 grains of rubble, lumps or chunks, or 1 bar' : '16 grains of rubble, lumps or chunks, or 2 bars') : costText(P.cost, 1);
+      const afford = P.kind === 'mold' || isWall(P.code) ? true : canAfford(P.cost, 1);
       const size = P.kind === 'machine' ? `${P.bw}×${P.bh}` : P.kind === 'mold' ? `${MOLDS[P.mold].bw}×${MOLDS[P.mold].bh}` : '1×1';
       h += `<div class="card"><div class="h"><span>${esc(P.name)}</span><span class="chip ${afford ? 'ok' : ''}">${P.power ? esc(P.power + ' power') : P.id === 'wheel' || P.id === 'engine' ? 'makes power' : size + (size === '1×1' ? ' block' : ' blocks')}</span></div><div class="p">${esc(P.desc)}</div><div class="p">Cost: ${esc(cost)}</div></div>`;
     }
@@ -58,7 +58,7 @@ function renderNotebook() {
 function controlsHtml() {
   const kb = [
     ['Fly', 'W A S D'], ['Boost while flying', 'Shift'], ['Tool modifier (while not flying)', 'Shift'], ['Use tool', 'Left mouse'], ['Second action', 'Right mouse'],
-    ['Tools', '1 – 4'], ['Select one to blow · piece', 'Q / E'], ['Switch build tab', 'Shift + Q / E'], ['Variant: material · direction · flip', 'R'], ['Wall or mold material menu (Build)', 'F'],
+    ['Tools', '1 – 4'], ['Select one to blow · piece', 'Q / E'], ['Switch build tab', 'Shift + Q / E'], ['Variant: material · direction · flip', 'R'], ['Sloped wall: turn · flip while dragging', 'T'], ['Wall or mold material menu (Build)', 'F'],
     ['Pick up selection or what you click (Pneumatic nozzle)', 'Left drag · Shift: everything'], ['Select (Pneumatic nozzle)', 'Click a bag name · Q/E'], ['Add to selection (Pneumatic nozzle)', 'Ctrl+click a bag name'], ['Show everything found (Pneumatic nozzle)', 'Hold Ctrl in the bag'], ['Clear selection (Pneumatic nozzle)', 'Clear in the bag'], ['Blow everything loose in the bag', 'Shift + right click'], ['Nozzle or brush size', '[ ]  or  Shift + wheel'],
     ['Replace what is built (Build)', 'Hold G'], ['Copy a box of what you built', 'Hold C, drag'], ['Cut a box (refunded, paste it back)', 'Hold X, drag'], ['Paste again', 'V'], ['Undo building', 'Ctrl + Z'], ['Blueprint library', 'B'], ['Zoom', 'Mouse wheel'], ['Interact (gates, molds, fans, filters)', 'F'], ['Thermal lens', 'H'], ['Info overlay: filters, belt and tube directions', 'I'], ['Bot lamp', 'L'],
     ['Pick the piece under the cursor (with its direction and settings)', 'Middle mouse'], ['Notebook', 'N'], ['Clear all blueprint ghosts', 'Delete'], ['Save', 'Ctrl + S'], ['Pause · menu', 'Esc'], ['Mute', 'Shift + M'], ['Full map', 'M'], ['Minimap zoom', '+ / − on minimap'],