feat(core): add tokenUsage to ChapterMeta schema

Optional field with promptTokens, completionTokens, totalTokens.
Backward-compatible — existing index.json files parse without it.

packages/core/src/models/chapter.ts

@@ -28,6 +28,11 @@ export const ChapterMetaSchema = z.object({
   detectionScore: z.number().min(0).max(1).optional(),
   detectionProvider: z.string().optional(),
   detectedAt: z.string().datetime().optional(),
+  tokenUsage: z.object({
+    promptTokens: z.number().int().default(0),
+    completionTokens: z.number().int().default(0),
+    totalTokens: z.number().int().default(0),
+  }).optional(),
 });
 
 export type ChapterMeta = z.infer<typeof ChapterMetaSchema>;