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>

docs/DESIGN.md

@@ -462,9 +462,10 @@ above, including:
   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.
+  belt has cleats: on 45° the load slides anyway. A downhill sloped belt is now a plain belt, with
+  no ribs and no sidewall, that carries at belt speed like a flat one (90 grains/s); only uphill
+  runs are sidewall belts. A whole run also shares one cleat phase, so pieces added later line up
+  with it.
 
 ## Ideas not built yet
 

docs/agent-log.md

@@ -4,6 +4,19 @@ What agents have done, newest first. Add an entry for every piece of work: the d
 note or request it was for, what changed, the files touched, the tests run, and anything left
 open or not verified. Keep each entry short; the git history has the details.
 
+## 2026-10-27: t42 feedback: downhill belts drop the sidewall too; new tasks (Claude Code)
+
+- Downhill sloped belts now draw as plain belts: no far wall and no ribs, placed or as ghosts or
+  drag previews (`slopeWallAt`, ghost drawing in `render/world.js`, `previewBlock` now gets the
+  belt direction from `previews.js` and `preview-selection.js`). `slopeUphill` takes the
+  direction bits instead of a block.
+- Test: `t_slope_belt_art.js` checks a downhill run draws no wall or ribs. Drag preview checked
+  headlessly with a counting canvas (downhill draws the band only); placed belts and ghosts
+  checked in screenshots. `python3 test/run_all.py --browser`: ALL PASSED. Rebuilt dist.
+- Filed t59 (world longevity audit), t60 (renewable resources, discussion), t61 (sloped walls
+  sticky after removing a piece mid-flow: not reproduced headless, see its report), t62 (screw
+  conveyor horizontal/diagonal joins), t63 (vacuum falling grains).
+
 ## 2026-10-26: t42 feedback: uphill stall at a seam, no cleats downhill, slope pick (Claude Code)
 
 - The owner's long uphill run stalled with a few grains, with a flashing seam between pieces 9