Implement the 002-add-combatant feature that adds the possibility to add new combatants to an encounter

specs/002-add-combatant/data-model.md

@@ -0,0 +1,77 @@
+# Data Model: Add Combatant
+
+**Feature**: 002-add-combatant
+**Date**: 2026-03-03
+
+## Entities
+
+### Combatant (existing, unchanged)
+
+| Field | Type | Constraints |
+|-------|------|-------------|
+| id | CombatantId (branded string) | Unique, required |
+| name | string | Non-empty after trimming, required |
+
+### Encounter (existing, unchanged)
+
+| Field | Type | Constraints |
+|-------|------|-------------|
+| combatants | readonly Combatant[] | Ordered list, may be empty |
+| activeIndex | number | 0 <= activeIndex < combatants.length (or 0 if empty) |
+| roundNumber | number | Positive integer >= 1, only increases |
+
+## Domain Events
+
+### CombatantAdded (new)
+
+| Field | Type | Description |
+|-------|------|-------------|
+| type | "CombatantAdded" (literal) | Discriminant for the DomainEvent union |
+| combatantId | CombatantId | Id of the newly added combatant |
+| name | string | Name of the newly added combatant |
+| position | number | Zero-based index where the combatant was placed |
+
+## State Transitions
+
+### AddCombatant
+
+**Input**: Encounter + CombatantId + name (string)
+
+**Preconditions**:
+- Name must be non-empty after trimming
+
+**Transition**:
+- New combatant `{ id, name: trimmedName }` appended to end of combatants list
+- activeIndex unchanged
+- roundNumber unchanged
+
+**Postconditions**:
+- combatants.length increased by 1
+- New combatant is at index `combatants.length - 1`
+- All existing combatants preserve their order and index positions
+- INV-2 satisfied (activeIndex still valid for the now-larger list)
+
+**Events emitted**: Exactly one `CombatantAdded`
+
+**Error cases**:
+- Empty or whitespace-only name → DomainError `{ code: "invalid-name" }`
+
+## Function Signatures
+
+### Domain Layer
+
+```
+addCombatant(encounter, id, name) → { encounter, events } | DomainError
+```
+
+### Application Layer
+
+```
+addCombatantUseCase(store, id, name) → DomainEvent[] | DomainError
+```
+
+## Validation Rules
+
+| Rule | Layer | Error Code |
+|------|-------|------------|
+| Name non-empty after trim | Domain | invalid-name |