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>

bgfilter/trimap.py

@@ -2,6 +2,7 @@ from __future__ import annotations
 
 import numpy as np
 
+from .chroma import bg_hue_projection
 from .deps import require_cv2
 from .settings import TrimapSettings
 
@@ -137,14 +138,8 @@ def fuse_trimap_directional(
     chroma_unknown = ~chroma_bg & ~chroma_fg
     core = seg_mask >= settings.seg_core_threshold
 
-    # Directional chroma: projection of (a*, b*) onto the background chroma
-    # direction. Positive = colour displaced toward the background hue.
-    lab_c = np.asarray(lab_center, dtype=np.float32)
-    mag = float(np.hypot(lab_c[1], lab_c[2]))
-    if mag < 1e-3:
-        proj = np.zeros(shape, dtype=np.float32)
-    else:
-        proj = (lab[..., 1] * lab_c[1] + lab[..., 2] * lab_c[2]) / mag
+    # Directional chroma: positive = colour displaced toward the background hue.
+    proj = bg_hue_projection(lab, lab_center)
     bg_hued = proj >= settings.bg_hue_proj_min
 
     bg = chroma_bg | ((seg_mask < settings.seg_low) & ~chroma_fg)