Shared join module for transport pieces; belt drawing uses it (t67 phase 1)

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

docs/agent-log.md

@@ -4,6 +4,22 @@ 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 1, shared join module (Claude Code + Codex)
+
+- Owner decision (DESIGN.md, replaces the t62 same-row rule): one join rule for every transport
+  piece. New DOM-free `src/sim/joins.js`: `pieceEnds`, `endJoin`, `pieceJoins`; rule tables are
+  compiled once and joins must be mutual. Belt drawing now uses it: `slopeBeltConnections`,
+  `flatBeltCaps` (now also for ghosts), and the pixel filter frame (`beltLike(b, end)`). Sloped
+  belt drag previews now pass a planned map.
+- Deliberate drawing changes where the old belt rules disagreed: flat belt caps no longer
+  treat any same-row diagonal screw as joined, and filter frames open only at a real join.
+- Files: `src/sim/joins.js`, `src/manifest.json`, `src/render/slope-belts.js`,
+  `src/render/setup.js`, `src/render/world.js`, `src/ui/previews.js`, `test/t_joins.js`,
+  `test/t_slope_belt_art.js`, `docs/PROJECT_MAP.md`, `docs/DESIGN.md`.
+- Tests: `python3 test/run_all.py --browser` all passed; t_joins checks 4,232 placements,
+  symmetry, planned maps, and that belt sand flow agrees with the rule. Sim output unchanged.
+- Open: screws still use their old rules until phase 2.
+
 ## 2026-10-27: t62 corner placements stay separate (Claude Code + Codex)
 
 - Owner tried the corner join and found the jog odd; decision (recorded in DESIGN.md): only