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>

README.md

@@ -9,6 +9,7 @@ RGB input
   -> chroma bg-confidence (keyed to the auto-detected or given colour)
   -> [optional] semantic segmentation mask
   -> trimap -> ViTMatte -> alpha cleanup
+  -> cross-model veto (second matting opinion on bg-hued residue)
   -> pymatting foreground -> despill -> RGBA PNG -> QA previews
 ```
 
@@ -28,6 +29,15 @@ Both share pymatting foreground estimation and a colour de-spill. There is **no*
 green-contamination rescue / recolour layer — with clean source images it is
 unnecessary, so it was removed.
 
+Both also run a **cross-model veto** by default: a second, trimap-free matting
+model (`ZhengPeng7/BiRefNet_HR-matting`) may only *lower* alpha, only on
+background-hued bright pixels the primary result is confident about — this clears
+colour-drifted background residue trapped between hair strands that the chroma
+key, the segmenter and ViTMatte all read as foreground. Costs one extra model
+download (~0.9 GB) and one inference pass per image; disable with
+`--no-cross-check` (see the `cross_check` config section, and
+`docs/hair_gap_artifacts.md` for the analysis behind it).
+
 The segmentation trimap defaults to `directional` mode (chroma + seg + a hue-direction
 split: it keeps a background-coloured garment such as a white shirt while dropping a
 background-hued residual such as blue trapped between hair strands). Switch with