Utilize `title_with_featured` for featured artist extraction and output enrichment

[shimpeiws]

Problem

Song.title_with_featured (e.g., "Song (ft. Drake & Future)") contains featured artist names but is never parsed or used. This info could improve section header matching accuracy in --filter-featured and enrich output.

Proposed Changes

New file: src/barscan/genius/featuring.py

• FEATURED_PATTERN regex matching ft., feat., featuring, with
• parse_featured_artists(title_with_featured) -> list[str]
• split_artist_names(names_str) -> list[str] - splits on &, ,, and

src/barscan/cli.py

• When --filter-featured is active, parse featured artists from song.title_with_featured
• Pass to analyze_text() for section matching

src/barscan/output/wordgrain.py

• Add featured_artists: tuple[str, ...] | None = Field(default=None) to WordGrainMeta
• Populate when available

Tests

• New tests/test_genius/test_featuring.py: various title formats, multiple artists, Japanese
• tests/test_output/test_wordgrain.py: featured_artists in meta
• tests/test_cli/test_commands.py: integration test

Dependencies

Depends on #57 (--filter-featured flag and featured_artists param from analyze_text()).