Implement the 008-persist-encounter feature that saves encounter state to localStorage so it survives page reloads

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

specs/008-persist-encounter/data-model.md

@@ -0,0 +1,50 @@
+# Data Model: Persist Encounter
+
+## Existing Entities (unchanged)
+
+### Combatant
+| Field | Type | Notes |
+|-------|------|-------|
+| id | CombatantId (branded string) | Unique identifier |
+| name | string | Display name |
+| initiative | number or undefined | Initiative value, optional |
+
+#### ID Format Convention (existing code)
+- Demo combatants use plain numeric IDs: `"1"`, `"2"`, `"3"`
+- User-added combatants use the pattern `c-{N}` (e.g., `"c-1"`, `"c-2"`)
+- On reload, `nextId` counter is derived by scanning existing IDs matching `c-{N}` and starting from `max(N) + 1`
+- IDs not matching `c-{N}` (e.g., demo IDs) are ignored during counter derivation
+
+### Encounter
+| Field | Type | Notes |
+|-------|------|-------|
+| combatants | readonly Combatant[] | Ordered list of combatants |
+| activeIndex | number | Index of the combatant whose turn it is |
+| roundNumber | number | Current round (positive integer) |
+
+## Persisted State
+
+### Storage Key
+`"initiative:encounter"`
+
+### Serialized Format
+The `Encounter` object is serialized as-is via `JSON.stringify`. The branded `CombatantId` serializes as a plain string. On deserialization, IDs are rehydrated with `combatantId()`.
+
+### Validation on Load
+1. Parse JSON string into unknown value
+2. Structural check: verify it is an object with `combatants` (array), `activeIndex` (number), `roundNumber` (number)
+3. Verify each combatant has `id` (string) and `name` (string)
+4. Pass through `createEncounter` to enforce domain invariants
+5. On any failure: discard and return `null` (caller falls back to demo encounter)
+
+### State Transitions
+
+```
+App Load
+  ├─ localStorage has valid data → restore Encounter
+  └─ localStorage empty/invalid  → create demo Encounter
+
+State Change (any use case)
+  └─ new Encounter saved to React state
+     └─ useEffect triggers → serialize to localStorage
+```