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>

src/cv/global_scan.py

@@ -580,13 +580,24 @@ def _prepare_motion_templates(
 
 
 def _is_dark_reference_frame(frame: np.ndarray, cfg: AppConfig) -> bool:
+    """Truly dark / pure-black frame: no usable structure for matching.
+
+    A cross-fade silhouette (low overall luma but visible contrast) is NOT
+    a dark frame for our purposes — it carries content (a hand, a knife,
+    a face peeking through the fade) and should still be matchable.
+    """
     cropped = text_safe_crop(
         frame,
         cfg.cv.vibe_check.crop_top_fraction,
         cfg.cv.vibe_check.crop_bottom_fraction,
     )
     gray = cv2.cvtColor(cropped, cv2.COLOR_BGR2GRAY)
-    return float(np.mean(gray)) < 28.0 and float(np.percentile(gray, 90)) < 58.0
+    mean = float(np.mean(gray))
+    p90 = float(np.percentile(gray, 90))
+    p10 = float(np.percentile(gray, 10))
+    contrast = p90 - p10
+    # Real darkness: low luma AND low contrast (no structure visible)
+    return mean < 28.0 and p90 < 58.0 and contrast < 30.0
 
 
 def _reference_visibility_stats(frame: np.ndarray, cfg: AppConfig) -> tuple[float, float, float]:
@@ -602,16 +613,40 @@ def _reference_visibility_stats(frame: np.ndarray, cfg: AppConfig) -> tuple[floa
 
 
 def _is_scoreable_reference_frame(frame: np.ndarray, cfg: AppConfig) -> bool:
-    """Exclude black, fade, and low-visibility reference frames from scoring."""
+    """Decide whether a reference frame can carry a usable match template.
+
+    Two acceptance paths:
+
+    * Standard: regular daylight / interior shot — luma at or above the
+      configured thresholds AND enough contrast to be distinct.
+    * Fade-content: low overall luma BUT with strong local contrast,
+      i.e. a cross-fade silhouette where you can clearly see structure
+      (hand+knife against dark, face emerging from black, etc.). Without
+      this path the matcher would silently drop content-bearing fades and
+      mis-match the visible portion alone.
+    """
     if _is_dark_reference_frame(frame, cfg):
         return False
 
     mean_luma, p90_luma, contrast = _reference_visibility_stats(frame, cfg)
-    low_visibility = (
-        mean_luma < cfg.cv.deep_scan.scoreable_luma_mean_min
-        and p90_luma < cfg.cv.deep_scan.scoreable_luma_p90_min
+
+    # Standard daylight / interior shot
+    enough_luma = (
+        mean_luma >= cfg.cv.deep_scan.scoreable_luma_mean_min
+        or p90_luma >= cfg.cv.deep_scan.scoreable_luma_p90_min
     )
-    return not low_visibility and contrast >= cfg.cv.deep_scan.scoreable_contrast_min
+    if enough_luma and contrast >= cfg.cv.deep_scan.scoreable_contrast_min:
+        return True
+
+    # Fade-content: dim but with structure. The local contrast must be
+    # well above what a uniform dim frame would have, and at least a few
+    # bright pixels must exist (p90 above pure-black), so we don't accept
+    # a featureless dark wash. These thresholds are deliberately tighter
+    # than the standard path so we don't pollute scoring with smooth fades.
+    if contrast >= 40.0 and p90_luma >= 30.0:
+        return True
+
+    return False
 
 
 def estimate_matchable_reference_duration(