Implement the 023-clear-encounter feature that adds a clear encounter button with confirmation dialog to remove all combatants and reset round/turn counters, with the cleared state persisting across page refreshes

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

specs/023-clear-encounter/data-model.md

@@ -0,0 +1,43 @@
+# Data Model: 023-clear-encounter
+
+## Entities
+
+### Encounter (existing — no schema changes)
+
+The `Encounter` type is unchanged. The clear operation produces an encounter with:
+
+| Field         | Cleared Value | Notes                          |
+|---------------|---------------|--------------------------------|
+| `combatants`  | `[]`          | Empty readonly array           |
+| `activeIndex` | `0`           | Reset to initial               |
+| `roundNumber` | `1`           | Reset to initial               |
+
+No new fields or types are added to the data model.
+
+## Events
+
+### EncounterCleared (new)
+
+| Field             | Type     | Description                                      |
+|-------------------|----------|--------------------------------------------------|
+| `type`            | `string` | Discriminant: `"EncounterCleared"`               |
+| `combatantCount`  | `number` | Number of combatants that were removed            |
+
+Minimal event shape — captures the count of cleared combatants for observability. No need to record the full pre-clear state (MVP baseline does not include undo).
+
+## State Transitions
+
+```
+clearEncounter(encounter: Encounter) → ClearEncounterSuccess | DomainError
+
+Input:  Any Encounter with combatants.length > 0
+Output: { encounter: { combatants: [], activeIndex: 0, roundNumber: 1 },
+          events: [EncounterCleared] }
+
+Error:  encounter.combatants.length === 0 → DomainError("encounter-already-empty")
+```
+
+## Persistence Impact
+
+- `saveEncounter()`: No changes — already serializes any `Encounter` to JSON.
+- `loadEncounter()`: Must handle `combatants: []` case by returning the empty encounter directly instead of routing through `createEncounter()` (which rejects empty combatant arrays).