Reading API types: narrow IWordCountService, seal records, separate API DTOs #1611

IWordCountService is wide (GetPageWordCount is just GetPageStats(...)?.PageWords ?? 0). ChapterWordCount/ReadingContextViewModel are unsealed records. ReadingProfileSummary is serialized as the public API contract, ChapterStatsResponse duplicates ChapterWordCount, and BookStatsResponse.cs holds two types. RecordSessionResult can represent both-null/both-set states.

Found during review of #1267; intentionally kept out of that PR.