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>

configs/default.yaml

@@ -1,3 +1,8 @@
+# Background-colour prior. null = auto-detect a green screen (default behaviour).
+# A hex string like "#CFEFFF" keys chroma + despill off that colour instead, for
+# arbitrary (e.g. pastel) flat backgrounds. Also overridable via --screen-color.
+screen_color: null
+
 model:
   model_name: hustvl/vitmatte-base-composition-1k
   device: cuda
@@ -5,11 +10,11 @@ model:
   fallback_to_chroma_alpha: false
 
 segmentation:
+  # enabled: true  = single-segmenter pipeline (one seg model drives the trimap)
+  # enabled: false = no seg model, chroma-only pipeline (Chroma + ViTMatte)
   enabled: true
-  backend: birefnet  # birefnet | anime-seg (anime-seg needs model_name: skytnt/anime-seg)
-  model_name: ZhengPeng7/BiRefNet
-  fill_backend: anime-seg          # dropout-fill uses anime-seg (higher wisp recall; add-only, harmless)
-  fill_model_name: skytnt/anime-seg
+  backend: anime-seg          # single trimap segmenter; switchable: anime-seg | birefnet
+  model_name: skytnt/anime-seg  # birefnet -> ZhengPeng7/BiRefNet
   device: cuda
   input_size: 1024
 
@@ -26,11 +31,6 @@ alpha_post:
   chroma_suppress_bg_low: 0.35
   chroma_suppress_bg_high: 0.80
   chroma_suppress_strength: 1.0
-  seg_fill: true
-  seg_fill_alpha_max: 0.10
-  seg_fill_seg_min: 0.10
-  seg_fill_strength: 0.70
-  seg_fill_min_area_ratio: 0.00002
 
 foreground:
   enabled: true
@@ -56,14 +56,4 @@ despill:
   green_excess_margin: 0.015
   edge_expand_radius: 2
   alpha_weight_floor: 0.35
-
-recolor:
-  enabled: true
-  person_threshold: 0.5
-  coverage_soft: 0.08
-  contam_low: 0.02
-  contam_high: 0.12
-  clean_dom_max: 0.03
-  propagate_radius_ratio: 0.02
-  min_reference_weight: 0.05
-  luminance_strength: 0.5
+  color_margin_lab: 4.0    # custom-screen despill: Lab chroma margin (screen_color set)