Applies Replit PR #21 feature on top of current main:
- buildWorld()/teardown() lifecycle for world rebuild on context restore
- disposeWorld(), disposeAgents(), disposeEffects(), disposeInteraction()
- getAgentStates()/applyAgentStates() for state preservation across rebuilds
- webgl-recovery-overlay in index.html
- Canvas reuse on reinit (existingCanvas param in initWorld)
- Preserves: visibility-change pause, visitor init, debounced resize