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/settings.py

@@ -5,6 +5,14 @@ from dataclasses import dataclass
 
 @dataclass(frozen=True)
 class ChromaSettings:
+    # False = non-flat-background mode (--no-chroma): no colour key at all.
+    # Segmentation alone drives the trimap (mode forced to "seg"), and 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 foreground
+    # fallback. Requires segmentation.enabled and matting_method 'vitmatte'.
+    enabled: bool = True
     border_ratio: float = 0.04
     min_samples: int = 2048
     lab_sigma_min: float = 10.0