Summary

• Adds renameAdversary(id:name:) -> Bool to EncounterSession
• Trims whitespace; rejects empty names and names already used as customName by another slot in the session
• Returns false without mutating state on any failure; @discardableResult for call sites that don't need the result

Notes

• Uses the full AdversaryState initializer directly — no changes needed to applying()
• Uniqueness is checked against customName only (not catalog base names), consistent with the field's semantics
• Required by Editable adversary name in runner with uniqueness validation encounter#68 (editable adversary names in the runner)

Test plan

• All 177 existing tests still pass (swift test)