Implement the 004-edit-combatant feature that adds the possibility to change a combatants name

specs/004-edit-combatant/contracts/domain-contract.md

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+# Domain Contract: editCombatant
+
+## Function Signature
+
+```
+editCombatant(encounter, id, newName) → EditCombatantSuccess | DomainError
+```
+
+### Inputs
+
+| Parameter | Type | Description |
+|-----------|------|-------------|
+| encounter | Encounter | Current encounter state |
+| id | CombatantId | Identity of combatant to rename |
+| newName | string | New name to assign |
+
+### Success Output
+
+| Field | Type | Description |
+|-------|------|-------------|
+| encounter | Encounter | Updated encounter with renamed combatant |
+| events | DomainEvent[] | Exactly one `CombatantUpdated` event |
+
+### Error Output
+
+| Code | Condition |
+|------|-----------|
+| `"combatant-not-found"` | No combatant with given id exists |
+| `"invalid-name"` | newName is empty or whitespace-only |
+
+## Hook Contract
+
+`useEncounter()` returns an additional action:
+
+| Method | Signature | Description |
+|--------|-----------|-------------|
+| editCombatant | `(id: CombatantId, newName: string) => void` | Rename combatant, append events on success |