Implement the 005-set-initiative feature that adds initiative values to combatants with automatic descending sort and active turn preservation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

specs/005-set-initiative/contracts/domain-api.md

@@ -0,0 +1,57 @@
+# Domain API Contract: Set Initiative
+
+## Function Signature
+
+```
+setInitiative(encounter, combatantId, value) → Success | DomainError
+```
+
+### Parameters
+
+| Parameter | Type | Description |
+|-----------|------|-------------|
+| encounter | Encounter | Current encounter state |
+| combatantId | CombatantId | Target combatant to update |
+| value | integer or undefined | New initiative value, or undefined to clear |
+
+### Success Result
+
+| Field | Type | Description |
+|-------|------|-------------|
+| encounter | Encounter | New encounter with updated combatant and reordered list |
+| events | DomainEvent[] | Array containing one `InitiativeSet` event |
+
+### Error Codes
+
+| Code | Condition |
+|------|-----------|
+| `combatant-not-found` | No combatant with the given id exists |
+| `invalid-initiative` | Value is defined but not an integer |
+
+### Ordering Contract
+
+After a successful call, `encounter.combatants` is sorted such that:
+1. All combatants with `initiative !== undefined` come before those with `initiative === undefined`
+2. Within the "has initiative" group: sorted descending by initiative value
+3. Within the "no initiative" group: original relative order preserved
+4. Equal initiative values: original relative order preserved (stable sort)
+
+### Active Turn Contract
+
+The combatant who was active before the call remains active after:
+- `encounter.activeIndex` points to the same combatant (by identity) in the new order
+- This holds even if the active combatant's own initiative changes
+
+### Invariants Preserved
+
+- INV-1: Empty encounters remain valid (0 combatants allowed)
+- INV-2: `activeIndex` remains in bounds after reorder
+- INV-3: `roundNumber` is never changed by `setInitiative`
+
+## Use Case Signature
+
+```
+setInitiativeUseCase(store, combatantId, value) → DomainEvent[] | DomainError
+```
+
+Follows the standard use case pattern: get encounter from store, call domain function, save on success, return events or error.