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/chroma.py

@@ -112,22 +112,6 @@ def _background_border_cluster(
     return full, share, agree
 
 
-def detect_background_color(
-    rgb: np.ndarray, settings: ChromaSettings | None = None
-) -> tuple[float, float, float]:
-    """Detect a single flat background colour from the image border.
-
-    Returns the colour as a 0..1 RGB triple. Raises RuntimeError (via
-    :func:`_background_border_cluster`) if the border is not dominated by one flat
-    colour, so callers fail loudly rather than matting against a wrong colour.
-    Pure: reads pixels only, with no matting side effects.
-    """
-    settings = settings or ChromaSettings()
-    rgb_f, lab = convert_color_spaces(rgb)
-    mask, _, _ = _background_border_cluster(lab, settings)
-    return tuple(float(x) for x in np.median(rgb_f[mask], axis=0))
-
-
 def estimate_background_model(
     rgb: np.ndarray,
     settings: ChromaSettings,