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>

configs/default.yaml

@@ -3,6 +3,13 @@
 # like "#CFEFFF" sets it explicitly. Also overridable via --screen-color.
 screen_color: null
 
+chroma:
+  # false = non-flat background mode (--no-chroma): no colour key; segmentation
+  # alone drives the trimap (mode forced to "seg") and every colour-keyed stage
+  # is bypassed (auto-detect, hue split, chroma suppression, despill, the
+  # cross-check veto). Needs segmentation.enabled and matting_method vitmatte.
+  enabled: true
+
 model:
   model_name: hustvl/vitmatte-base-composition-1k
   device: cpu