Edit mode: pre-fill `AdversaryCreatorForm` from existing local adversary

[gwillish]

Per redesign-plan.md M7. Depends on all four form step issues (#41–44) and #140.

AdversaryCreatorForm accepts an optional existing Adversary parameter. When provided:

• All four steps are pre-filled with the existing adversary's data
• Save overwrites the existing record (via ContentStore.saveLocalAdversary)

This is the "Edit" path triggered from AdversaryInspectorView (#131) for local adversaries.

Test first: AdversaryCreatorFormTests — edit mode pre-fills name, type, tier, stats, features; Save updates the existing adversary without creating a duplicate.