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/settings.py

@@ -42,20 +42,6 @@ class AlphaPostSettings:
     chroma_suppress_bg_low: float = 0.35
     chroma_suppress_bg_high: float = 0.80
     chroma_suppress_strength: float = 1.0
-    # Segmentation dropout-fill: rescue alpha where ViTMatte cut a region the
-    # semantic mask asserts is foreground (green-contaminated wisps it cannot see).
-    seg_fill: bool = True
-    seg_fill_alpha_max: float = 0.10
-    seg_fill_seg_min: float = 0.10
-    seg_fill_strength: float = 0.70
-    seg_fill_min_area_ratio: float = 0.00002  # drop rescued specks below this area
-    # Intersection fill: gate the (anime-seg) dropout-fill by a second, reliable
-    # mask (BiRefNet). anime-seg proposes freely at a low seg_fill_seg_min; only
-    # rescues BiRefNet also endorses (>= seg_fill_gate_min, its threshold left
-    # unchanged) survive. Trims anime-seg's over-inclusive halo to the structure
-    # both backends agree on, without reintroducing BiRefNet's grainy value.
-    seg_fill_intersect: bool = False
-    seg_fill_gate_min: float = 0.10
 
 
 @dataclass(frozen=True)
@@ -85,19 +71,10 @@ class DespillSettings:
     green_excess_margin: float = 0.015
     edge_expand_radius: int = 2
     alpha_weight_floor: float = 0.35
-
-
-@dataclass(frozen=True)
-class RecolorSettings:
-    enabled: bool = True
-    person_threshold: float = 0.5      # clean reference = subject above this seg/alpha
-    coverage_soft: float = 0.08        # recolour ramps in over alpha 0..this
-    contam_low: float = 0.02           # residual off-colour where recolour starts
-    contam_high: float = 0.12          # residual off-colour for full recolour
-    clean_dom_max: float = 0.03        # reference = subject pixels below this off-colour
-    propagate_radius_ratio: float = 0.02
-    min_reference_weight: float = 0.05
-    luminance_strength: float = 0.5    # how far to pull luminance to the clean neighbour
+    # Custom-screen despill (when screen_color is set): margin, in Lab a*/b*
+    # units, of chroma allowed along the background colour direction before it is
+    # treated as spill. Larger keeps more of the subject's own colour.
+    color_margin_lab: float = 4.0
 
 
 @dataclass(frozen=True)
@@ -110,15 +87,11 @@ class ModelSettings:
 
 @dataclass(frozen=True)
 class SegmentationSettings:
-    enabled: bool = True
-    backend: str = "birefnet"  # "birefnet" or "anime-seg"
-    model_name: str = "ZhengPeng7/BiRefNet"
-    fill_backend: str = ""     # "" = reuse `backend` for dropout-fill; else e.g. "anime-seg"
-    fill_model_name: str = ""
+    enabled: bool = True       # False = chroma-only pipeline (no seg model)
+    backend: str = "anime-seg"  # "anime-seg" or "birefnet"
+    model_name: str = "skytnt/anime-seg"
     device: str = "cuda"
     input_size: int = 1024
-    sharpen: float = 0.0       # unsharp-mask amount on the mask (0 = off)
-    sharpen_radius: int = 4
 
 
 @dataclass(frozen=True)
@@ -128,6 +101,9 @@ class PipelineSettings:
     alpha_post: AlphaPostSettings = AlphaPostSettings()
     foreground: ForegroundSettings = ForegroundSettings()
     despill: DespillSettings = DespillSettings()
-    recolor: RecolorSettings = RecolorSettings()
     model: ModelSettings = ModelSettings()
     segmentation: SegmentationSettings = SegmentationSettings()
+    # Background-colour prior. None = auto-detect a green screen (current default,
+    # unchanged). A hex string like "#CFEFFF" keys chroma + despill off that colour
+    # instead, for arbitrary (e.g. pastel) flat backgrounds.
+    screen_color: str | None = None