Implement the 003-remove-combatant feature that adds the possibility to remove a combatant from an encounter

specs/003-remove-combatant/data-model.md

@@ -0,0 +1,69 @@
+# Data Model: Remove Combatant
+
+**Feature**: 003-remove-combatant
+**Date**: 2026-03-03
+
+## Existing Entities (no changes)
+
+### Encounter
+
+| Field | Type | Description |
+|-------|------|-------------|
+| combatants | readonly Combatant[] | Ordered list of participants |
+| activeIndex | number | Index of the combatant whose turn it is |
+| roundNumber | number | Current round (≥ 1, never changes on removal) |
+
+### Combatant
+
+| Field | Type | Description |
+|-------|------|-------------|
+| id | CombatantId (branded string) | Unique identifier |
+| name | string | Display name |
+
+## New Event Type
+
+### CombatantRemoved
+
+| Field | Type | Description |
+|-------|------|-------------|
+| type | "CombatantRemoved" (literal) | Event discriminant |
+| combatantId | CombatantId | ID of the removed combatant |
+| name | string | Name of the removed combatant |
+
+Added to the `DomainEvent` discriminated union alongside `TurnAdvanced`, `RoundAdvanced`, and `CombatantAdded`.
+
+## New Domain Function
+
+### removeCombatant
+
+| Parameter | Type | Description |
+|-----------|------|-------------|
+| encounter | Encounter | Current encounter state |
+| id | CombatantId | ID of combatant to remove |
+
+**Returns**: `RemoveCombatantSuccess | DomainError`
+
+### RemoveCombatantSuccess
+
+| Field | Type | Description |
+|-------|------|-------------|
+| encounter | Encounter | Updated encounter after removal |
+| events | DomainEvent[] | Exactly one CombatantRemoved event |
+
+### DomainError (existing, reused)
+
+Returned with code `"combatant-not-found"` when ID does not match any combatant.
+
+## State Transition Rules
+
+### activeIndex Adjustment
+
+Given removal of combatant at index `removedIdx` with current `activeIndex`:
+
+| Condition | New activeIndex |
+|-----------|----------------|
+| removedIdx > activeIndex | activeIndex (unchanged) |
+| removedIdx < activeIndex | activeIndex - 1 |
+| removedIdx === activeIndex, not last in list | activeIndex (next slides in) |
+| removedIdx === activeIndex, last in list | 0 (wrap) |
+| Only combatant removed (list becomes empty) | 0 |