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/config.py

@@ -7,6 +7,7 @@ from typing import Any, TypeVar
 from .settings import (
     AlphaPostSettings,
     ChromaSettings,
+    CrossCheckSettings,
     DespillSettings,
     ForegroundSettings,
     ModelSettings,
@@ -48,7 +49,7 @@ def _update_dataclass(instance: T, values: dict[str, Any] | None) -> T:
 def settings_from_dict(data: dict[str, Any]) -> PipelineSettings:
     data = dict(data)
     screen_color = data.pop("screen_color", None)
-    allowed_sections = {"chroma", "trimap", "alpha_post", "foreground", "despill", "model", "segmentation"}
+    allowed_sections = {"chroma", "trimap", "alpha_post", "cross_check", "foreground", "despill", "model", "segmentation"}
     unknown_sections = sorted(set(data) - allowed_sections)
     if unknown_sections:
         raise ValueError(f"Unknown config section(s): {', '.join(unknown_sections)}")
@@ -57,6 +58,7 @@ def settings_from_dict(data: dict[str, Any]) -> PipelineSettings:
         chroma=_update_dataclass(ChromaSettings(), data.get("chroma")),
         trimap=_update_dataclass(TrimapSettings(), data.get("trimap")),
         alpha_post=_update_dataclass(AlphaPostSettings(), data.get("alpha_post")),
+        cross_check=_update_dataclass(CrossCheckSettings(), data.get("cross_check")),
         foreground=_update_dataclass(ForegroundSettings(), data.get("foreground")),
         despill=_update_dataclass(DespillSettings(), data.get("despill")),
         model=_update_dataclass(ModelSettings(), data.get("model")),
@@ -100,6 +102,9 @@ def override_settings(settings: PipelineSettings, **overrides: Any) -> PipelineS
     despill_updates: dict[str, Any] = {}
     if overrides.get("despill") is not None:
         despill_updates["enabled"] = overrides["despill"]
+    cross_check_updates: dict[str, Any] = {}
+    if overrides.get("cross_check") is not None:
+        cross_check_updates["enabled"] = overrides["cross_check"]
     seg_updates: dict[str, Any] = {}
     if overrides.get("device") is not None:
         seg_updates["device"] = overrides["device"]
@@ -121,6 +126,7 @@ def override_settings(settings: PipelineSettings, **overrides: Any) -> PipelineS
         chroma=chroma,
         trimap=_update_dataclass(trimap, trimap_updates),
         alpha_post=alpha_post,
+        cross_check=_update_dataclass(settings.cross_check, cross_check_updates),
         foreground=foreground,
         despill=_update_dataclass(despill, despill_updates),
         model=_update_dataclass(model, model_updates),