Add cross-model veto of hair-gap background residue (default on)

Colour-drifted background trapped between hair strands defeats every
single-signal defence: the chroma key reads it as foreground (bgc ~0.07),
the segmenter backs it, ViTMatte rates it opaque, and post-hoc removal is
a proven dead end (it shreds the hair volume the same pixels belong to).
A second, trimap-free matting model (BiRefNet_HR-matting) is the only
tested model that separates this residue from the subject, so its opinion
is fused in as a veto: min-fusion that may only LOWER alpha, restricted to
the background-hued bright suspect zone (proj >= 3, L >= 45, feathered)
and gated by primary-alpha confidence (0.70 -> 0.95 ramp) so soft wisps
and dark hair are exempt by construction.

- settings/config/CLI: cross_check block, --cross-check/--no-cross-check
- alpha_post.cross_check_alpha after clean_alpha; second opinion reuses
  BiRefNetSegmenter; saved to debug as cross_check_alpha.png
- chroma.bg_hue_projection extracted and shared with the trimap
- docs: methodology.md (new), hair_gap_artifacts.md (investigation log)

Verified: cross-check ON reproduces the visually-reviewed B1gate
prototype byte-for-byte on TestImage3; --no-cross-check reproduces the
previous baseline byte-for-byte; pink-bg FixImage1 face untouched.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

configs/default.yaml

@@ -46,6 +46,22 @@ alpha_post:
   suppress_raw_lo: 0.85
   suppress_raw_hi: 0.98
 
+cross_check:
+  # Cross-model veto: a second, trimap-free matting model may only LOWER alpha
+  # (min-fusion), only on background-hued bright pixels the primary result is
+  # confident about — clears colour-drifted background residue between hair
+  # strands that chroma, segmentation and ViTMatte all read as foreground.
+  # Costs one extra model (~0.9 GB download) and one inference pass per image.
+  # Disable with enabled: false or --no-cross-check.
+  enabled: true
+  model_name: ZhengPeng7/BiRefNet_HR-matting
+  input_size: 2048
+  proj_min: 3.0        # bg-hue projection above which a pixel is suspect
+  l_min: 45.0          # Lab lightness below which a pixel is exempt (dark hair)
+  feather_sigma: 2.0   # zone-boundary feather, px
+  gate_lo: 0.70        # primary alpha below this -> fully exempt
+  gate_hi: 0.95        # primary alpha above this -> fully vetoable
+
 foreground:
   enabled: true
   method: ml