feat(inventory): implement v2 data structure and JSDoc types (Epic 7.1)

Create structured inventory system with categorized storage:
- Add JSDoc type definitions (InventoryV2, InventoryV1, MigrationResult)
- Implement migration utility for v1 → v2 conversion
- Update state.js with v2 inventory structure and feature flag
- Update config.js to match new inventory defaults

Changes:
- NEW: src/types/inventory.js (30 lines) - Type definitions
- NEW: src/utils/migration.js (84 lines) - Migration logic
- MODIFIED: src/core/state.js - v2 inventory + FEATURE_FLAGS
- MODIFIED: src/core/config.js - v2 inventory defaults

Inventory v2 structure:
{
  version: 2,
  onPerson: "Sword, 3x Potions",     // Plaintext string
  stored: {
    "Home": "Clothes, Tools",         // Location → items
    "Bank": "Gold, Documents"
  },
  assets: "Motorcycle, House"         // Plaintext string
}

Migration handles all edge cases:
- v1 string → v2.onPerson
- null/undefined → v2 defaults
- "None" → v2 defaults
- Already v2 → no changes

Context overhead: +40 chars (~5% increase) for organized multi-location storage

Part of Epic 7: New Inventory System
Implements: docs/IMPLEMENTATION_PLAN.md Epic 7.1

src/utils/migration.js

@@ -0,0 +1,84 @@
+/**
+ * Inventory Migration Module
+ * Handles conversion from v1 (string) to v2 (structured) inventory format
+ */
+
+// Type imports
+/** @typedef {import('../types/inventory.js').InventoryV1} InventoryV1 */
+/** @typedef {import('../types/inventory.js').InventoryV2} InventoryV2 */
+/** @typedef {import('../types/inventory.js').MigrationResult} MigrationResult */
+
+/**
+ * Default v2 inventory structure for new/empty inventories
+ * @type {InventoryV2}
+ */
+const DEFAULT_INVENTORY_V2 = {
+    version: 2,
+    onPerson: "None",
+    stored: {},
+    assets: "None"
+};
+
+/**
+ * Migrates inventory data from v1 (string) to v2 (structured) format.
+ * Handles all edge cases: null, undefined, "None", already-migrated data.
+ *
+ * @param {InventoryV1 | InventoryV2 | null | undefined} inventory - Inventory data to migrate
+ * @returns {MigrationResult} Migration result with v2 inventory and metadata
+ */
+export function migrateInventory(inventory) {
+    // Case 1: Already v2 format (has version property and is an object)
+    if (inventory && typeof inventory === 'object' && inventory.version === 2) {
+        // console.log('[RPG Companion Migration] Inventory already v2, no migration needed');
+        return {
+            inventory: inventory,
+            migrated: false,
+            source: 'v2'
+        };
+    }
+
+    // Case 2: null or undefined → use defaults
+    if (inventory === null || inventory === undefined) {
+        // console.log('[RPG Companion Migration] Inventory is null/undefined, using defaults');
+        return {
+            inventory: { ...DEFAULT_INVENTORY_V2 },
+            migrated: true,
+            source: 'null'
+        };
+    }
+
+    // Case 3: v1 string format → migrate to v2
+    if (typeof inventory === 'string') {
+        // Check if it's an empty/default string
+        const trimmed = inventory.trim();
+        if (trimmed === '' || trimmed.toLowerCase() === 'none') {
+            // console.log('[RPG Companion Migration] Inventory is empty/None, using defaults');
+            return {
+                inventory: { ...DEFAULT_INVENTORY_V2 },
+                migrated: true,
+                source: 'v1'
+            };
+        }
+
+        // Non-empty v1 string → migrate to v2.onPerson
+        // console.log('[RPG Companion Migration] Migrating v1 string to v2.onPerson:', inventory);
+        return {
+            inventory: {
+                version: 2,
+                onPerson: inventory,
+                stored: {},
+                assets: "None"
+            },
+            migrated: true,
+            source: 'v1'
+        };
+    }
+
+    // Case 4: Unknown format (malformed object, number, etc.) → use defaults
+    console.warn('[RPG Companion Migration] Unknown inventory format, using defaults:', inventory);
+    return {
+        inventory: { ...DEFAULT_INVENTORY_V2 },
+        migrated: true,
+        source: 'default'
+    };
+}