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/core/state.js

@@ -3,6 +3,9 @@
  * Centralizes all extension state variables
  */
 
+// Type imports
+/** @typedef {import('../types/inventory.js').InventoryV2} InventoryV2 */
+
 /**
  * Extension settings - persisted to SillyTavern settings
  */
@@ -40,7 +43,13 @@ export let extensionSettings = {
         arousal: 0,
         mood: '😐',
         conditions: 'None',
-        inventory: 'None'
+        /** @type {InventoryV2} */
+        inventory: {
+            version: 2,
+            onPerson: "None",
+            stored: {},
+            assets: "None"
+        }
     },
     classicStats: {
         str: 10,
@@ -94,6 +103,13 @@ export let isPlotProgression = false;
  */
 export let pendingDiceRoll = null;
 
+/**
+ * Feature flags for gradual rollout of new features
+ */
+export const FEATURE_FLAGS = {
+    useNewInventory: true // Enable v2 inventory system with categorized storage
+};
+
 /**
  * Fallback avatar image (base64-encoded SVG with "?" icon)
  * Using base64 to avoid quote-encoding issues in HTML attributes