chore(vv): clarify execution tracing as verification and animation visualisation as validation

Author: amyheather

pages/guide/verification_validation/verification_validation.qmd

@@ -109,7 +109,7 @@ You'll probably perform debugging naturally as you code the model - but it is he
 
 ### Execution tracing
 
-@Balci1998 describes **execution tracing** as watching the model outputs, following the detailed sequence of events, state changes, and decisions as the model runs, and checking them against what you expect.
+@Balci1998 describes **execution tracing** as watching the model outputs, following the detailed sequence of events, state changes, and decisions as the model runs, and checking them against what you expect. We are comparing against the conceptual model / expected logic.
 
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@@ -369,7 +369,7 @@ Predictive validation is a requirement in the DES reporting checklist from @Zhan
 
 ### Animation visualisation
 
-If you build an animation that shows your simulation model as it runs, it can help you spot errors by letting you "see" what's happening (@Balci1998). You can watch patients flow, queues grow and shrink, and resources being used - and you maybe be able to spot odd behaviours.
+If you build an animation that shows your simulation model as it runs, it can help you spot errors by letting you "see" what's happening (@Balci1998). You can watch patients flow, queues grow and shrink, and resources being used - and you maybe be able to spot odd behaviours. Animation can also support verification by revealing obvious implementation bugs, but here we emphasise its use for assessing whether behaviour 'looks like' the real system.
 
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