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

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

src/render/setup.js

@@ -116,8 +116,8 @@ function thinPipeColor(x, y, anim) {
   }
   PC[0] = r; PC[1] = g; PC[2] = bl;
 }
-// a block that is a belt or a filter: they join up into one belt
-function beltLike(b) { if (b < 0 || b >= NB) return false; const c = blockP[b]; return isBelt(c) || isFilter(c); }
+// A filter frame opens only where this end has a geometric partner.
+function beltLike(b, end) { return endJoin(b % BW, (b / BW) | 0, end) >= 0; }
 // standing trees: the trunk shows its dark bark, so it reads apart from the pale cut wood it drops
 const TREE_R = new Uint8Array(NMAT), TREE_G = new Uint8Array(NMAT), TREE_B = new Uint8Array(NMAT);
 for (let m = 0; m < NMAT; m++) { TREE_R[m] = COLR[m]; TREE_G[m] = COLG[m]; TREE_B[m] = COLB[m]; }