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>

configs/birefnet.yaml

@@ -55,7 +55,6 @@ despill:
   enabled: true
   edge_low: 0.005
   strength: 0.92
-  green_excess_margin: 0.015
   edge_expand_radius: 2
   alpha_weight_floor: 0.35
   color_margin_lab: 4.0