Downhill sloped belts draw as plain belts, ghosts and previews too

Downhill runs have no cleats, so they no longer draw the sidewall or
ribs either: placed, as ghosts, or in the drag preview, which now knows
the belt's direction. Uphill runs keep the wall and ribs.

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

test/t_slope_belt_art.js

@@ -137,6 +137,9 @@ check('far wall is filled down to the belt and grains cover it',run(`(()=>{
   }
   return wall>20;
 })()`));
+flatWorld(run,160,120,112);
+run('for(let k=0;k<6;k++)placeBlock(BP_BELT_UP,0,12+k,20-k,true);rebuildPower();slopeCleatsTick(false);prepareSlopeBelts(0,0,W,H);');
+check('a downhill belt draws no wall and no ribs',run('(()=>{let n=0;for(let i=0;i<N;i++)if(slopeDetailAt(i,i%W,(i/W)|0))n++;return n===0 && cleatCells.length===0;})()'));
 flatWorld(run,320,240,232);
 run('for(let k=0;k<8;k++)placeBlock(BP_BELT_UP,1,12+k,30-k,true);for(let t=0;t<120;t++)simTick();');
 check('idle empty slope lets every moving area sleep',run('awake.every(v=>v===0)'),run('awake.reduce((n,v)=>n+(v>0),0)'));