Fix matching regressions, cache guard, and multi-shot algorithm for beat 15

- config.toml: revert scoreable_luma/contrast thresholds to 24/58/24 (lowering
  them let cross-fade blend frames contaminate content-validation templates,
  dropping scores below provisional_content_threshold)
- src/cv/global_scan.py: _is_dark_reference_frame now requires contrast<30 so
  genuine dark silhouette frames are not rejected as scoreable; two-path
  _is_scoreable_reference_frame separates standard vs fade-content scoring
- cli.py: _keeps_cached_match() guard prevents a weaker single-span rematch
  from overwriting a better multi-segment provisional cache entry
- cli.py: _fade_content_shots() restricted to between-island gaps only—
  pre-island black leaders were incorrectly emitted as matchable shots
- cli.py: island[0] of _match_unmatched_visual_segments() now uses no
  continuity seed so an insert cut at the start of a multi-shot beat is not
  forced toward the previous beat's scene
- scripts/generate_cutter_report.py: fix ffmpeg concat demuxer on Windows—
  use part.absolute().as_posix() so paths in the concat txt are absolute and
  not double-resolved relative to the concat file's directory

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/generate_cutter_report.py

@@ -219,7 +219,7 @@ def extract_concat_clip(
     # encoder settings).
     list_file = out.with_name(f"{out.stem}_concat.txt")
     list_file.write_text(
-        "\n".join(f"file '{part.as_posix()}'" for part in parts) + "\n",
+        "\n".join(f"file '{part.absolute().as_posix()}'" for part in parts) + "\n",
         encoding="utf-8",
     )
     cmd = [