fix(core): stabilize collinear wall junction ordering

packages/core/src/systems/wall/wall-mitering.test.ts

@@ -79,6 +79,21 @@ describe('wall miter boundary sides', () => {
   })
 })
 
+describe('deterministic junction ordering', () => {
+  test('same wall set produces the same miter data regardless of input order', () => {
+    const thickWall = (id: string, thickness: number, end: [number, number]) =>
+      ({ ...wall(id, [0, 0], end), thickness }) as WallNode
+    const wide = thickWall('wide', 0.6, [4, 0])
+    const narrow = thickWall('narrow', 0.2, [4, 0])
+    const branch = thickWall('branch', 0.4, [0, 4])
+
+    const forward = calculateLevelMiters([wide, narrow, branch]).junctionData.get('0,0')
+    const reversed = calculateLevelMiters([narrow, wide, branch]).junctionData.get('0,0')
+
+    expect(forward).toEqual(reversed)
+  })
+})
+
 describe('junction grid prefilter', () => {
   function thickWall(
     id: string,

packages/core/src/systems/wall/wall-mitering.ts

@@ -357,8 +357,13 @@ function calculateJunctionIntersections(
     }
   }
 
-  // Sort by outgoing angle
-  processedWalls.sort((a, b) => a.angle - b.angle)
+  // Sort by outgoing angle, then by wall ID so equal-angle walls produce the
+  // same pairing regardless of scene iteration order.
+  processedWalls.sort((a, b) => {
+    const angleOrder = a.angle - b.angle
+    if (angleOrder !== 0) return angleOrder
+    return a.wallId < b.wallId ? -1 : a.wallId > b.wallId ? 1 : 0
+  })
 
   const wallIntersections = new Map<string, { left?: Point2D; right?: Point2D }>()
   const n = processedWalls.length