Editable adversary name in runner with uniqueness validation

[gwillish]

Summary

Allow the GM to rename an adversary slot during a live encounter. Names must be unique across all slots in the session.

Requirements

• Editing is accessible from the expanded AdversaryRunnerCard — e.g. tapping the name in the card header makes it inline-editable
• On commit, validate that the new name is not already used by another slot in the session; show an inline error if it is
• Empty names are not allowed; if the field is cleared, revert to the previous value
• The renamed slot's display updates immediately in both the collapsed row and the expanded card
• Mutation routes through EncounterSession (a new method, e.g. renameAdversary(id:name:)) — the view does not write to AdversaryState directly

Depends on

• Auto-assign numbered names for duplicate adversaries at session creation DHModels#17 (auto-assign at creation — establishes the customName field as the source of truth for display)
• gwillish/DHModels needs a renameAdversary(id:name:) method on EncounterSession (can be added in the same PR as Code review cleanup: isHomebrew, index-based ForEach, plist fix #17 or separately)