Remove dead colour-model/despill code; simplify despill; refresh stale docs

- chroma: drop BackgroundModel.hsv_center/hue_sigma (unused since the green gating
  went away), their estimation, _hue_distance, and the now-unused hsv channel from
  convert_color_spaces (one fewer cvtColor per image).
- despill: delete the dead green-channel-algebra path (the pipeline always passes a
  colour) and fold the remaining Lab-direction despill into a single despill() with
  no screen_color arg (127 -> 60 lines). Renamed despill_green -> despill.
- settings: remove the now-dead ChromaSettings.hue_sigma_min and
  DespillSettings.green_excess_margin (plus the matching config lines).
- docs/comments: generalise stale "green" wording to "background colour" (module
  docstring, alpha_post/trimap/foreground/settings) and rename the trimap variable
  green_bg -> screen_bg.

Kept: the unmix foreground fallback, --matting-method chroma, and green/blue as
illustrative examples. Verified: package compiles, both configs parse, and an
auto-detect end-to-end run is unchanged.

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

bgfilter/settings.py

@@ -7,7 +7,6 @@ from dataclasses import dataclass
 class ChromaSettings:
     border_ratio: float = 0.04
     min_samples: int = 2048
-    hue_sigma_min: float = 7.0
     lab_sigma_min: float = 10.0
     rgb_sigma_min: float = 0.08
     # Background-colour auto-detection (used when screen_color is None): find the
@@ -50,7 +49,7 @@ class AlphaPostSettings:
     alpha_floor: float = 0.002
     alpha_ceil: float = 0.998
     # Chroma-guided alpha suppression: in the unknown band, pull alpha toward 0
-    # where background (green) confidence is high, clearing spill in gaps/holes.
+    # where background-colour confidence is high, clearing spill in gaps/holes.
     chroma_suppress: bool = True
     chroma_suppress_bg_low: float = 0.35
     chroma_suppress_bg_high: float = 0.80
@@ -81,12 +80,11 @@ class DespillSettings:
     enabled: bool = True
     edge_low: float = 0.005
     strength: float = 0.92
-    green_excess_margin: float = 0.015
     edge_expand_radius: int = 2
     alpha_weight_floor: float = 0.35
-    # 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.
+    # Margin, in Lab a*/b* units, of chroma allowed along the background-colour
+    # direction before it counts as spill. Larger keeps more of the subject's own
+    # colour.
     color_margin_lab: float = 4.0