feat: primary-clip variant detection, commentary specials, disc8/disc9 fixtures

[yxbh]

Summary

Add primary-clip variant detection, commentary special detection via IG menu analysis, and ICS fallback loading for test fixtures. Add disc8 and disc9 fixtures with integration tests.

Background

Some Blu-ray discs include multiple playlists for the same content with different audio tracks or short ending clips. These were not caught by exact signature dedup because their full segment keys differ. This caused disc9 (single-movie disc) to be incorrectly analyzed as 2 episodes + 0 specials instead of 1 episode + 1 special.

Additionally, disc menus can list audio commentary tracks as special features — these are IG menu buttons that JumpTitle to episode playlists (not separate playlists). Without IG analysis, these were invisible to the special feature detector.

Changes

Primary-clip variant detection

• bdpl/analyze/signatures.py: Add find_primary_clip_variants() — groups playlists sharing the same dominant first clip (>85% of total duration) with similar timing
• bdpl/analyze/__init__.py: Integrate variant detection into dedup pipeline (step 1b); add _maybe_collapse_variant_episodes() to prevent chapter splitting when variant dedup reduces episode count to one

Commentary special detection (new)

• bdpl/analyze/__init__.py: _detect_special_features() now identifies IG menu buttons that JumpTitle to episode playlists from non-chapter-selection pages as commentary specials
• Chapter-selection pages (all buttons target same title with different reg2 chapter indices) are excluded from commentary detection to avoid false positives
• Stream-variant playlists are skipped using the existing variant_mpls set

ICS fallback loading (new)

• bdpl/analyze/__init__.py: _parse_ig_hints() now falls back to loading ics_menu.bin from the BDMV path when no STREAM/*.m2ts files are available — enables IG-based analysis in test fixtures without bundling media streams

Disc8 fixture

• tests/fixtures/disc8/: 7 MPLS, 11 CLPI, index.bdmv, MovieObject.bdmv, META/DL/bdmt_eng.xml, ics_menu.bin
• tests/test_disc8_scan.py: 2 episodes (4 scenes each), 3 specials (2 commentaries + 1 lyrics ending)

Disc9 fixture

• tests/fixtures/disc9/: 4 MPLS, 9 CLPI, index.bdmv, MovieObject.bdmv, META/DL/bdmt_eng.xml
• tests/test_disc9_scan.py: 1 episode (movie ~90min), 1 special (alt-audio variant)

Shared changes

• tests/conftest.py: Add disc8/disc9 path and analysis fixtures
• tests/test_disc_matrix.py: Add disc8/disc9 to all 6 matrix parametrizations

Testing

• ruff check . — all checks passed
• ruff format . — all files formatted
• pytest -q — 186 tests passed (up from 154)
• All existing disc1–disc7 test expectations unchanged

Additional Notes

• Disc8 specials: 3 (2 commentaries referencing episode playlists + 1 lyrics credit ending on 00005.mpls; 00006.mpls is a stream variant excluded by dedup)
• Disc9: variant detection correctly groups 00002.mpls + 00003.mpls as duplicates; 00002 becomes the representative episode, 00003 becomes special
• Disc2 ICS fallback loads 27 IG hints but produces no false commentary detections (all JumpTitle targets are on chapter-selection pages)
• All fixture titles are generic per copyright guidelines