Add recolor pass to clean residual contamination colour

Add bgfilter/recolor.py: after despill, propagate clean neighbour chroma into
pixels whose colour is still off (residual green, or the magenta a green unmix
overshoots into), keeping luminance partway so hair texture survives. It installs
a real observed colour rather than subtracting, so it cannot overshoot. Despill
already neutralises the matte edge (off ~ 0 there), so recolor only touches the
contaminated interior and the wisps the dropout-fill rescued.

Runs as a final colour pass on the despilled foreground; pymatting + despill keep
their stronger edge handling. On the samples the magenta fringe drops sharply
(TestImage vis-magenta 0.065->0.014, TestImage2 0.085->0.031) with edge green
unchanged (vis-green ~0.02). Debug output color_mask.png replaces the despill /
foreground-background / foreground-correction maps; smoke updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bgfilter/config.py

@@ -11,6 +11,7 @@ from .settings import (
     ForegroundSettings,
     ModelSettings,
     PipelineSettings,
+    RecolorSettings,
     SegmentationSettings,
     TrimapSettings,
 )
@@ -43,7 +44,7 @@ def _update_dataclass(instance: T, values: dict[str, Any] | None) -> T:
 
 
 def settings_from_dict(data: dict[str, Any]) -> PipelineSettings:
-    allowed_sections = {"chroma", "trimap", "alpha_post", "foreground", "despill", "model", "segmentation"}
+    allowed_sections = {"chroma", "trimap", "alpha_post", "foreground", "despill", "recolor", "model", "segmentation"}
     unknown_sections = sorted(set(data) - allowed_sections)
     if unknown_sections:
         raise ValueError(f"Unknown config section(s): {', '.join(unknown_sections)}")
@@ -54,6 +55,7 @@ def settings_from_dict(data: dict[str, Any]) -> PipelineSettings:
         alpha_post=_update_dataclass(AlphaPostSettings(), data.get("alpha_post")),
         foreground=_update_dataclass(ForegroundSettings(), data.get("foreground")),
         despill=_update_dataclass(DespillSettings(), data.get("despill")),
+        recolor=_update_dataclass(RecolorSettings(), data.get("recolor")),
         model=_update_dataclass(ModelSettings(), data.get("model")),
         segmentation=_update_dataclass(SegmentationSettings(), data.get("segmentation")),
     )
@@ -71,6 +73,7 @@ def override_settings(settings: PipelineSettings, **overrides: Any) -> PipelineS
     alpha_post = settings.alpha_post
     foreground = settings.foreground
     despill = settings.despill
+    recolor = settings.recolor
     model = settings.model
     segmentation = settings.segmentation
 
@@ -102,6 +105,7 @@ def override_settings(settings: PipelineSettings, **overrides: Any) -> PipelineS
         alpha_post=alpha_post,
         foreground=foreground,
         despill=_update_dataclass(despill, despill_updates),
+        recolor=recolor,
         model=_update_dataclass(model, model_updates),
         segmentation=_update_dataclass(segmentation, seg_updates),
     )