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>

docs/DESIGN.md

@@ -461,6 +461,10 @@ above, including:
   scoops what lies on the feeding belt behind it into its pocket, like a real boot section.
   Measured: about 80-86 grains/s uphill under a heavy feed (flat 90); downhill about 38, since
   the top only feeds a layer two grains deep onto the face.
+- **No cleats on downhill belts (t42 feedback, 2026-10-26).** The owner asked why a downhill
+  belt has cleats: on 45° the load slides anyway. A downhill sloped belt is now a plain belt with
+  a sidewall that carries at belt speed like a flat one (90 grains/s); only uphill runs have
+  ribs. A whole run also shares one cleat phase, so pieces added later line up with it.
 
 ## Ideas not built yet