Joins follow the travel direction: the next piece goes right after the end (t67 phase 2b)

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

docs/agent-log.md

@@ -4,6 +4,25 @@ 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: t67 phase 2b, joins follow the travel direction (Claude Code + Codex)
+
+- The owner tested phase 2: horizontal → uphill and vertical-up → diagonal didn't join like the
+  rest. New rule (DESIGN.md): a piece's outlet points at its block plus its travel vector, and
+  the next piece joins when it sits there travelling the same general way. Belts bend it where
+  grains can't follow: an uphill belt's top feeds a flat belt on the same row, and a flat belt
+  one block below an uphill low end still joins.
+- `src/sim/joins.js` reworked around inlets and outlets; screw drawing, belt drawing, screw
+  grain flow and bars follow it. Straight-screw mouths no longer reach a diagonal across a
+  corner; straight-to-straight transfers (lift side-feeding) unchanged.
+- Files: `src/sim/joins.js`, `src/sim/conveyors.js`, `src/sim/items.js`,
+  `src/render/diagonal-screws.js`, `src/render/slope-belts.js`, `src/render/slope-belt-shader.js`,
+  `src/render/world.js`, `src/ui/previews.js`, `tools/scenes/joins.js`, tests `t_joins.js`,
+  `t_diag_screws.js`, `t_slope_belt_art.js`, `t_smooth.py`, `docs/PROJECT_MAP.md`.
+- Tests: `python3 test/run_all.py --browser` all passed.
+- Open: Codex hit its usage limit before finishing. A flat belt feeding an uphill belt on the
+  same row is joined but draws badly in the smooth look (overlapping casing, stepped edges;
+  `shots/joins-belt-drop-smooth.png`). Not playtested; determinism diff not re-run by hand.
+
 ## 2026-10-27: t67 phase 2, screw conveyors follow the join rule (Claude Code + Codex)
 
 - Screw drawing, grain flow and bars now all go through `endJoin`. New joins: a horizontal in