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

specs/004-edit-combatant/data-model.md

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+# Data Model: Edit Combatant
+
+**Feature**: 004-edit-combatant
+**Date**: 2026-03-03
+
+## Entities
+
+### Combatant (unchanged)
+
+| Field | Type | Notes |
+|-------|------|-------|
+| id | CombatantId (branded string) | Immutable identity |
+| name | string | Mutable — this feature updates it |
+
+### Encounter (unchanged structure)
+
+| Field | Type | Notes |
+|-------|------|-------|
+| combatants | readonly Combatant[] | Edit replaces name in-place by mapping |
+| activeIndex | number | Preserved during edit |
+| roundNumber | number | Preserved during edit |
+
+## Events
+
+### CombatantUpdated (new)
+
+| Field | Type | Notes |
+|-------|------|-------|
+| type | "CombatantUpdated" | Discriminant |
+| combatantId | CombatantId | Which combatant was renamed |
+| oldName | string | Name before edit |
+| newName | string | Name after edit |
+
+Added to the `DomainEvent` union type.
+
+## State Transitions
+
+### editCombatant(encounter, id, newName)
+
+**Preconditions**:
+- `newName` is non-empty and not whitespace-only
+- `id` matches a combatant in `encounter.combatants`
+
+**Postconditions**:
+- The combatant with matching `id` has `name` set to `newName`
+- `activeIndex` and `roundNumber` unchanged
+- Combatant list order unchanged
+- Exactly one `CombatantUpdated` event emitted
+
+**Error cases**:
+- `id` not found → `DomainError { code: "combatant-not-found" }`
+- `newName` empty/whitespace → `DomainError { code: "invalid-name" }`
+
+## Validation Rules
+
+| Rule | Condition | Error Code |
+|------|-----------|------------|
+| Name must be non-empty | `newName.trim().length === 0` | `"invalid-name"` |
+| Combatant must exist | No combatant with matching `id` | `"combatant-not-found"` |