sync: update core and viewer from monorepo (#143)

Major changes:
- Roof system rewrite with roof-segment support
- Scene store refactor
- Spatial grid improvements
- Item light system
- Post-processing and selection manager updates
- Perf monitor component

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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

@@ -95,12 +95,12 @@ function findJunctions(walls: WallNode[]): Map<string, Junction> {
     if (!junctions.has(keyStart)) {
       junctions.set(keyStart, { meetingPoint: startPt, connectedWalls: [] })
     }
-    junctions.get(keyStart)!.connectedWalls.push({ wall, endType: 'start' })
+    junctions.get(keyStart)?.connectedWalls.push({ wall, endType: 'start' })
 
     if (!junctions.has(keyEnd)) {
       junctions.set(keyEnd, { meetingPoint: endPt, connectedWalls: [] })
     }
-    junctions.get(keyEnd)!.connectedWalls.push({ wall, endType: 'end' })
+    junctions.get(keyEnd)?.connectedWalls.push({ wall, endType: 'end' })
   }
 
   // Second pass: detect T-junctions (walls passing through junction points)

packages/core/src/systems/wall/wall-system.tsx

@@ -46,7 +46,7 @@ export const WallSystem = () => {
       if (!dirtyWallsByLevel.has(levelId)) {
         dirtyWallsByLevel.set(levelId, new Set())
       }
-      dirtyWallsByLevel.get(levelId)!.add(id)
+      dirtyWallsByLevel.get(levelId)?.add(id)
     })
 
     // Process each level that has dirty walls