Sidewall belt and screen drawing, cheaper cleat checks (t42, t43)

Far sidewall drawn behind the load with cleat ribs at their physics
positions; sloped screens get a tinted mesh between rails and shake only
when loaded. Powder skips cleat checks outside chunks that have cleats,
and empty slopes let their chunks sleep. Record the screw conveyor and
stall/carry-depth decisions.

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

docs/agent-log.md

@@ -4,6 +4,32 @@ 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.
 
+## 2026-10-26: t42/t43 drawing and performance follow-up (Codex)
+
+- Filled the far sidewall down to the belt, behind grains, with two-tone pixel corrugation
+  and a continuous smooth 45-degree band. Rubber ribs use the physics mask in all eight
+  phases. Fixed smooth wall seams; matched ghosts, cursor previews and menu icons.
+  Smooth screens retain tinted mesh, now between solid rails without the pixel overlay;
+  loaded-only shake and transport are unchanged.
+- Empty cleats only wake nearby load; per-chunk flags bypass mask reads for distant powder.
+  Empty eight-block slope: 3 -> 0 moving areas after 120 ticks. Uphill sand/gravel/copper ore
+  71.3 grains/s, coal 71.2; exact sorting retained. Isolated 1f8a89d -> current, median five
+  generated falling-world samples: no slopes 2.006 -> 2.063 ms/tick; 50 slopes 1.952 -> 2.076.
+  Powder probe with cached zero flag: 45.02 -> 50.56 ms (2.14M calls); residual 12 percent
+  microbenchmark overhead, about 3 percent in the generated no-slope world. Existing loaded
+  576-block stress: 0.515 -> 1.064 ms/tick. slope-cost.js records the reproducible methodology;
+  raw results /tmp/t42-perf-{pre,after}-isolated.txt. Pre-t42 loaded slopes use old grip physics.
+- Files: sim cells/slope-conveyors/world; render slope-belts/slope-belt-shader/world;
+  UI previews/build-menu; t_slope_belt_art.js, t_smooth.py, slope-cost.js; PROJECT_MAP,
+  DESIGN and t42/t43 reports. Reused enterFree for the ordinary entry rules.
+- Final python3 test/run_all.py --browser: ALL PASSED (26 headless suites, 360 play,
+  16 smooth checks; art suite 57). Before/after suite outputs differ only by five new
+  checks (/tmp/t42-output.diff). Opened shots/t{42,43}-follow-{belt,screen}-{pixel,smooth}-z{4,8}.png;
+  rechecked smooth belt after fixing seams; opened t42-follow-ghosts-{pixel,smooth}-z8.png
+  with both updated menu icons visible. Added sleep, border/load wake, wall geometry,
+  grain colour and smooth grain-class checks. Rebuilt dist, regenerated map; both tasks Review.
+  Owner playtest and hardware GPU appearance remain unverified. No commit.
+
 ## 2026-10-26: t42: sidewall belts with moving cleats (Codex)
 
 - Continued the uncommitted prototype. `sim/slope-conveyors.js` supplies moving barrier