Simplify to single-segmenter pipelines; add screen_color prior; drop rescue + recolor

Strategic pivot: the dropout-fill rescue, the anime-seg fill backend, the
BiRefNet+anime-seg intersection, and the recolor pass all existed to repair
source images whose hair was already green-contaminated or broken at generation
time. Fixing the source instead (clean pastel-background generation) makes the
matte high-contrast, so that whole compensation layer is unnecessary. Remove it.

- Two pipelines via segmentation.enabled:
    false -> chroma-only (Chroma + ViTMatte)
    true  -> single segmenter (default anime-seg, switchable birefnet) -> trimap
  then ViTMatte refines, pymatting estimates foreground, despill cleans spill.

- Generalise the green-hardcoded colour logic to a screen_color prior (hex, e.g.
  "#CFEFFF"; default null = green auto-detect, byte-identical). chroma keys off
  Lab/RGB distance to the colour; despill removes chroma along its Lab direction.
  Added --screen-color CLI flag.

- Removed dead code: fill_seg_dropouts + seg_fill*/intersection params,
  SegmentationSettings.fill_backend/fill_model_name/sharpen, unsharp_mask,
  recolor.py + RecolorSettings.

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

bgfilter/config.py

@@ -11,7 +11,6 @@ from .settings import (
     ForegroundSettings,
     ModelSettings,
     PipelineSettings,
-    RecolorSettings,
     SegmentationSettings,
     TrimapSettings,
 )
@@ -44,7 +43,9 @@ 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", "recolor", "model", "segmentation"}
+    data = dict(data)
+    screen_color = data.pop("screen_color", None)
+    allowed_sections = {"chroma", "trimap", "alpha_post", "foreground", "despill", "model", "segmentation"}
     unknown_sections = sorted(set(data) - allowed_sections)
     if unknown_sections:
         raise ValueError(f"Unknown config section(s): {', '.join(unknown_sections)}")
@@ -55,9 +56,9 @@ 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")),
+        screen_color=screen_color,
     )
 
 
@@ -73,7 +74,6 @@ 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
 
@@ -99,13 +99,18 @@ def override_settings(settings: PipelineSettings, **overrides: Any) -> PipelineS
     if overrides.get("device") is not None:
         seg_updates["device"] = overrides["device"]
 
+    screen_color = (
+        overrides["screen_color"]
+        if overrides.get("screen_color") is not None
+        else settings.screen_color
+    )
     return PipelineSettings(
         chroma=chroma,
         trimap=_update_dataclass(trimap, trimap_updates),
         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),
+        screen_color=screen_color,
     )