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/pipeline.py

@@ -8,7 +8,7 @@ import numpy as np
 
 from .alpha_post import clean_alpha, enforce_trimap, suppress_alpha_by_chroma
 from .chroma import compute_bg_confidence, parse_hex_color
-from .despill import despill_green
+from .despill import despill
 from .foreground import estimate_foreground_rgb
 from .io import load_rgb, save_gray, save_rgb, save_rgba, write_text
 from .qa import BACKGROUND_COLORS, composite, make_qa_grid, save_previews
@@ -83,9 +83,6 @@ def _run_image(
     bg_confidence, model = compute_bg_confidence(
         rgb, settings=settings.chroma, screen_color=screen_color
     )
-    # With no colour given, chroma auto-detected one (model.rgb_center); reuse it for
-    # despill so both key off the same background colour instead of the green path.
-    despill_color = screen_color if screen_color is not None else tuple(model.rgb_center)
     if settings.segmentation.enabled:
         seg_mask = pipeline._segment(rgb)
         mode = settings.trimap.mode
@@ -94,7 +91,7 @@ def _run_image(
         elif mode in ("directional", "directional-hard-bg"):
             from .chroma import convert_color_spaces
 
-            lab = convert_color_spaces(rgb)[2]
+            lab = convert_color_spaces(rgb)[1]
             trimap, trimap_stats = fuse_trimap_directional(
                 seg_mask, bg_confidence, lab, model.lab_center, settings.trimap
             )
@@ -114,8 +111,8 @@ def _run_image(
     foreground = estimate_foreground_rgb(
         rgb, alpha, bg_confidence, model, settings.foreground
     )
-    corrected_rgb, color_mask = despill_green(
-        foreground.rgb, alpha, model, settings.despill, screen_color=despill_color
+    corrected_rgb, color_mask = despill(
+        foreground.rgb, alpha, model, settings.despill
     )
     save_rgba(output_path, corrected_rgb, alpha)