Auto-detect the background colour (default); drop the green heuristic

Replace the hardcoded green-screen auto-detection with a general flat-colour
detector. When screen_color is null the background colour is now detected from the
image border (dominant colour of the border strip) and used directly as the chroma
model, with a failure gate that raises when the border is not one clean flat colour
(gradient / texture / subject filling the frame).

- chroma: add detect_background_color + _background_border_cluster; the null case of
  estimate_background_model samples the detected cluster directly (no seed search);
  compute_bg_confidence is now always perceptual (Lab/RGB distance). Removed the green
  heuristic (initial_green_candidates), the green multiplicative gating, the now-unused
  _smoothstep, and five green-only ChromaSettings fields.
- pipeline: reuse the detected colour for de-spill in the auto case.
- settings / default.yaml: add detect_* tuning fields; refresh screen_color docs.
- README: document auto-detection + the failure gate, trimap modes, birefnet config.

A supplied --screen-color hex still uses the seed-search refinement. Validated:
detects green / pastel / text samples correctly, raises on gradient / two-colour, CLI
exits 1 on failure, green and pastel mattes unchanged in quality.

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

bgfilter/pipeline.py

@@ -83,6 +83,9 @@ 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
@@ -112,7 +115,7 @@ def _run_image(
         rgb, alpha, bg_confidence, model, settings.foreground
     )
     corrected_rgb, color_mask = despill_green(
-        foreground.rgb, alpha, model, settings.despill, screen_color=screen_color
+        foreground.rgb, alpha, model, settings.despill, screen_color=despill_color
     )
     save_rgba(output_path, corrected_rgb, alpha)