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

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

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.