Uphill runs share one cleat phase; no cleats downhill; pick slope bands

A piece added to a running sloped belt kept its own cleat phase, so at
the seam a rib could start just behind another and push its pocket into
it, stalling the whole run with only a few grains. Every block of a run
now takes the phase of the run's lower-left block.

Downhill sloped belts no longer have cleats: they carry like a flat belt
(90 grains/s, was 38) and draw only the sidewall.

Middle click on a sloped belt's band, where it is drawn into the empty
block beside it, now picks the belt.

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

test/t_slope_belt_art.js

@@ -99,7 +99,7 @@ check('drawing leaves the seed, cells and saved block data unchanged', run(`(()
 })()`));
 for (const grade of [-1,1]) {
   flatWorld(run,160,120,112);
-  run(`for(let k=0;k<6;k++)placeBlock(${grade<0?'BP_BELT_UP':'BP_BELT_DOWN'},1,12+k,${grade<0?20:12}+k*${grade},true);`);
+  run(`for(let k=0;k<6;k++)placeBlock(${grade<0?'BP_BELT_UP':'BP_BELT_DOWN'},${grade<0?1:0},12+k,${grade<0?20:12}+k*${grade},true);`); // uphill, the cleated way
   for(let phase=0;phase<8;phase++) {
     run(`rebuildPower();slopePhase.fill(${phase});slopeCleatsTick(false);prepareSlopeBelts(0,0,W,H);`);
     check(`grade ${grade} phase ${phase} draws exactly the physical cleat raster`,run(`(()=>{