Add non-flat background mode (--no-chroma / chroma.enabled: false)

No colour key at all: segmentation alone drives the trimap (mode forced
to 'seg', whose chroma terms degrade to no-ops on a zero background-
confidence map), ViTMatte still refines the unknown band at full
resolution, and pymatting still estimates edge foreground colour. Every
colour-keyed stage is bypassed: background auto-detect, the directional
hue split, chroma alpha suppression, despill, the cross-check veto (its
suspect zone is background-hued by definition; with reuse_as_seg the
second opinion still serves as the segmenter), and the unmix fallback.

Guards: requires segmentation.enabled and matting_method 'vitmatte'
(clear errors otherwise); despill/foreground handle model=None.

Validation: flat-background default path is bit-identical pre/post
(alpha and RGB |D|max = 0 on TestImage3). On a known-alpha subject
composited over a gradient+blotch background, recovered alpha scores
MAE 0.0018 / IoU@0.5 0.993 (0.995 with --cross-check-as-seg); the same
input correctly fails auto-detection in default mode.

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

bgfilter/despill.py

@@ -10,7 +10,7 @@ from .settings import DespillSettings
 def despill(
     rgb: np.ndarray,
     alpha: np.ndarray,
-    model: BackgroundModel,
+    model: BackgroundModel | None,
     settings: DespillSettings,
 ) -> tuple[np.ndarray, np.ndarray]:
     """Remove background-colour spill from the foreground.
@@ -24,7 +24,8 @@ def despill(
 
     Returns the corrected RGB (uint8) and the per-pixel despill weight (float32).
     """
-    if not settings.enabled:
+    # model is None with chroma disabled: no key colour means no spill direction.
+    if not settings.enabled or model is None:
         return rgb.copy(), np.zeros(alpha.shape, dtype=np.float32)
 
     bg_lab = np.asarray(model.lab_center, dtype=np.float32)