Shared drag placement; drags snap one block to join a diagonal (t67 phases 3-4)

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

test/t_slope_belts.js

@@ -219,6 +219,11 @@ for(const id of ['belt','filter'])for(const [dx,dy] of [[3,-3],[3,3],[-3,3],[-3,
   check(`${id} a single replacement keeps the selected direction`,run(`blockP[${20+dy}*BW+${20+dx}]===${c} && blockV[${20+dy}*BW+${20+dx}]===${id==='filter'?v:1-v}`));
 
 }
+// The owner's drag: a flat belt, then an uphill drag started beside its end on the same level,
+// snaps up one block through the real release so the two join seamlessly.
+fresh();
+run("globalThis.piece=()=>PIECE_BY_ID.belt;placeBlock(BP_BELT,1,19,20,true);GAME.drag={bx:20,by:20};GAME.aim={x:23*4,y:17*4};bot.x=80;bot.y=80;primaryRelease(false);");
+check('uphill drag beside a flat belt end snaps up one and joins it', run('blockP[20*BW+20]===0 && blockP[19*BW+20]===BP_BELT_UP && blockP[16*BW+23]===BP_BELT_UP && endJoin(19,20,1)>=0 && !joinIsDrop(19,20,1,endJoin(19,20,1))'));
 // Middle click on the belt band you see, even where it is drawn into the empty block next door.
 fresh();
 const band=run(`(()=>{globalThis.R=globalThis.R||{smoothOn:false};