Remove dead code: unused detect_background_color and ForegroundEstimate extras

- chroma.detect_background_color had no callers anywhere (auto-detection
  goes through estimate_background_model / _background_border_cluster).
- ForegroundEstimate.background and .correction were computed on every
  image but never consumed; only .rgb was used. estimate_foreground_rgb
  now returns the foreground array directly and the dataclass is gone,
  which also drops pymatting return_background work and a per-image
  correction-norm pass.

Verified: CLI output on samples/TestImage3.png is byte-identical
(SHA256) to the pre-cleanup baseline.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bgfilter/pipeline.py

@@ -178,7 +178,7 @@ def _process_rgb(rgb: np.ndarray, pipeline: MattingPipeline) -> MattingResult:
     foreground = estimate_foreground_rgb(
         rgb, alpha, bg_confidence, model, settings.foreground
     )
-    corrected_rgb, color_mask = despill(foreground.rgb, alpha, model, settings.despill)
+    corrected_rgb, color_mask = despill(foreground, alpha, model, settings.despill)
 
     metadata = {
         "alpha_source": alpha_source,