fix: align README and help text with actual implementation (issue #75)

[ShadyUnderLight]

Summary

Fix documentation inconsistencies described in Issue #75.

Changes

1. Cover art fetch order (README.md)

• Line 15 and lines 188-194 now correctly reflect actual code order:
  1. Embedded cover (priority, no network)
  2. MusicBrainz / Cover Art Archive
  3. iTunes Search API
  4. Local directory image
  5. LeftFM (disabled by default)

2. Project structure section (README.md)

• Removed non-existent EmbeddedChapterReader.swift and TextChapterParser.swift
• Updated ChapterSource.swift description to note it handles all chapter parsing
• Updated AlbumArtFetcher.swift description to match actual order

3. CLI help text (CLI/main.swift)

• Changed {index:02d} to {index} — the formatter syntax is not actually supported by AudioSplitter.applyNameTemplate()

Verification

• Swift build succeeds
• 108 tests pass