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

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

docs/DESIGN.md

@@ -486,11 +486,16 @@ above, including:
   on warns first that the save will be marked, and the mark stays with the world; any future
   achievements won't unlock in a marked world. Besides money, items and upgrades it may have
   game speed and other things just for fun.
-- **Screw conveyors only join a diagonal on the same row (t62, 2026-10-27).** A straight screw
-  joins a diagonal when it sits right beside the diagonal's end block, and that join is drawn
-  as an elbow. When the diagonal instead starts one block lower or higher, so the two only touch
-  corner to corner, they stay two separate ends and nothing passes between them. The owner tried
-  a corner join and found it looked odd, and building it that way is unusual anyway.
+- **One join rule for every transport piece (2026-10-27, replaces the t62 same-row rule).** The
+  owner wants all pieces to place and join the same way, so players know what to expect when
+  building. At a diagonal piece's end, call E the block diagonally past its last block (the
+  block its end points into). Another diagonal of the same slope continues in E. A horizontal
+  piece (flat belt, flat filter, horizontal screw) joins on the same row at the high end, and
+  in E, one block lower, at the low end: the rule sloped belts already had. A vertical screw
+  joins in E at either end. Straight pieces join the next block along their axis. Any other
+  placement is two separate ends and nothing passes between them. The owner found the old
+  screw rule, which needed the horizontal placed over the diagonal's last block at the low
+  end, inconsistent with every other piece.
 
 ## Ideas not built yet
 

docs/PROJECT_MAP.md

@@ -118,6 +118,13 @@ dist/, shots/         build output and screenshots (not committed)
   - kind `mold`.
   `hidden` pieces are not in the menu. The menu's tabs and order are `BUILD_CATS` in
   `src/ui/build-menu.js`.
+- **Transport geometry:** `joins.js` owns geometric ends and joins for belts, filters and screws.
+  `pieceEnds(code, v)` returns precomputed left/right ends (vertical screws: up/down).
+  `endJoin(bx, by, endIndex, planned = null)` returns `neighbourBlock * 2 + neighbourEnd`,
+  or -1; `pieceJoins(bx, by, planned = null)` returns bits 0/1 for the joined ends.
+  Plans override world blocks, including the queried block. Continuing diagonals take priority
+  over elbows; a join is returned only when both ends choose each other. Belt drawing uses
+  this module; screw drawing and movement still use their existing rules.
 - **Placing and removing:**
   - `placeBlock` / `removeBlock` (`blocks.js`);
   - `placeMachineAt` / `addMachine` / `removeMachine` (`actions.js`, `machines.js`);
@@ -265,6 +272,11 @@ Pipes, pumps and outlets: each joined line holds one liquid as a count, not as g
 Per-tick machine work: steam engines, fans, belts, filters, screw conveyors (machinesTick).
 - functions: engineIntake, fanBlow, machinesTick
 
+### src/sim/joins.js (71 lines)
+Geometric transport ends. Tables are built once; querying a world or plan allocates nothing. Ends are left/right, except vertical screws (up/down). Travel direction is irrelevant.
+- functions: pieceEnds, joinCandidate, endJoin, pieceJoins
+- variables: JOIN_ENDS
+
 ### src/sim/conveyors.js (198 lines)
 How belts, filters and screw conveyors move what is on or in them. What rests on a belt or filter rides along: the whole stack standing on the carrying surface steps one cell toward d together (belts run front of the line first, so the way ahead is clear). If a powder grain is blocked, the load above it stands still and the belt slips underneath. A filter's own material drops through it instead, unless it is already full.
 - functions: conveyorStep, carryColumn, beltDrain, launchGrain, rebuildFilters, filterCarry, carryBeltsAndFilters, setFilter, filterRun, roomFor, screwSpread, screwIntake, diagonalScrewNext
@@ -362,7 +374,7 @@ Drawing (src/render/): browser only. This file: the renderer state R, colour tab
 Propagated coloured light from glowing and burning cells.
 - functions: builtForLight, looseForLight, computeLight
 
-### src/render/slope-belts.js (176 lines)
+### src/render/slope-belts.js (157 lines)
 Drawing only: the face stays on its simulation cells; the band hangs below it.
 - functions: slopeBeltConnections, flatBeltCaps, prepareSlopeBelts, slopeBeltSegment, slopeBeltCorner, slopeBeltShape, slopeBeltAt, slopeBeltColor, screenLoaded, slopeWallShape, slopeWallAt, slopeDetailAt, slopePreviewCleat
 - variables: slopeBeltJoins, slopeBeltHalo, beltCaps, slopeAcross, slopeAlong, slopeDistance, slopeWallOwner
@@ -372,7 +384,7 @@ Screw casing geometry shared by both looks, ghosts and placement previews. smoot
 - functions: diagonalScrewJoins, diagonalScrewShape, diagonalScrewBlock
 - variables: SCREW_CLASS, diagonalJoins, diagonalAcross, diagonalAlong, diagonalCap
 
-### src/render/world.js (402 lines)
+### src/render/world.js (403 lines)
 Scale colour without changing the smooth-look class.
 - functions: fogColor, drawWorld
 
@@ -530,6 +542,7 @@ The frame loop and boot.
 - `test/t_econ.js`: Economy: the dock (instant sale), orders, prices, the market dip, money for machines, alloys.
 - `test/t_fog.js`: Fog: surface, sight, range, saves and simulation independence.
 - `test/t_furnace_repeat.js`: The starter furnaces keep working when fired again and again without cleaning, however the charge goes in: mixed, ore first with coal on top, or blown in through the vent. Each case plays furnace_play.js, which builds the design and fires it like a player.
+- `test/t_joins.js`: The oracle states the geometric rule independently of the precomputed neighbour tables.
 - `test/t_light.js`: Run the renderer's lighting against real simulation data, with only canvas storage stubbed.
 - `test/t_load.js`: Reload wakes nearby areas first, without losing distant water, fire or saved state.
 - `test/t_molds.js`: Molds and furnaces stay usable: ingots drop below and keep their stubs, plugs over molds are cleared, spoiled castings can be knocked out, grains never drop through a grate by teleporting, and slag lets the fire breathe.

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