Fix: SoA diff serializer correctly handles undefined values for entities without components

[americanjeff]

Summary

• Fixed hasChanged function in SoASerializer to properly handle undefined array slots when entities don't have all components
• Fixed shadow array initialization to not fill with zeros (which caused false change detection for undefined slots)

Root Cause

When using diff-mode serialization over all entities (e.g., getAllEntities()), entities that don't have a particular component have undefined slots in that component's arrays. The getShadow function was initializing shadow arrays with 0, and hasChanged was treating undefined vs undefined as a change (returning true instead of false).

Changes

1. getShadow: Remove .fill(0) - shadow arrays now initialize with undefined slots
2. hasChanged: Add proper handling for undefined values:
   • Both undefined → no change, return false
   • Shadow undefined, current defined → first time seeing value, return true
   • Current undefined, shadow defined → entity lost component, return true

Test Plan

All existing tests pass (162 tests).