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/data-model.md

@@ -0,0 +1,63 @@
+# Data Model: Set Initiative
+
+## Entity Changes
+
+### Combatant (modified)
+
+| Field | Type | Required | Description |
+|-------|------|----------|-------------|
+| id | CombatantId (branded string) | Yes | Unique identifier |
+| name | string | Yes | Display name (non-empty, trimmed) |
+| initiative | integer | No | Initiative value for turn ordering. Unset means "not yet rolled." |
+
+**Validation rules**:
+- `initiative` must be an integer when set (no floats, NaN, or Infinity)
+- Zero and negative integers are valid
+- Unset (`undefined`) is valid — combatant has not rolled initiative yet
+
+### Encounter (unchanged structure, new ordering behavior)
+
+| Field | Type | Required | Description |
+|-------|------|----------|-------------|
+| combatants | readonly Combatant[] | Yes | Ordered list. Now sorted by initiative descending (unset last, stable sort for ties). |
+| activeIndex | number | Yes | Index of the active combatant. Adjusted to follow the active combatant's identity through reorders. |
+| roundNumber | number | Yes | Current round (≥ 1). Unchanged by initiative operations. |
+
+**Ordering invariant**: After any `setInitiative` call, `combatants` is sorted such that:
+1. Combatants with initiative come first, ordered highest to lowest
+2. Combatants without initiative come last
+3. Ties within each group preserve relative insertion order (stable sort)
+
+## New Domain Event
+
+### InitiativeSet
+
+Emitted when a combatant's initiative value is set, changed, or cleared.
+
+| Field | Type | Description |
+|-------|------|-------------|
+| type | "InitiativeSet" | Event discriminant |
+| combatantId | CombatantId | The combatant whose initiative changed |
+| previousValue | integer or undefined | The initiative value before the change |
+| newValue | integer or undefined | The initiative value after the change |
+
+## State Transitions
+
+### setInitiative(encounter, combatantId, value)
+
+**Input**: Current encounter, target combatant id, new initiative value (integer or undefined to clear)
+
+**Output**: Updated encounter with reordered combatants and adjusted activeIndex, plus events
+
+**Error conditions**:
+- `combatant-not-found`: No combatant with the given id exists in the encounter
+- `invalid-initiative`: Value is not an integer (when defined)
+
+**Transition logic**:
+1. Find target combatant by id → error if not found
+2. Validate value is integer (when defined) → error if invalid
+3. Record the active combatant's id (for preservation)
+4. Update the target combatant's initiative value
+5. Stable-sort combatants: initiative descending, unset last
+6. Find the active combatant's new index in the sorted array
+7. Return new encounter + `InitiativeSet` event