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/config.py

@@ -99,6 +99,9 @@ def override_settings(settings: PipelineSettings, **overrides: Any) -> PipelineS
     }
     if overrides.get("trimap_mode") is not None:
         trimap_updates["mode"] = overrides["trimap_mode"]
+    chroma_updates: dict[str, Any] = {}
+    if overrides.get("chroma") is not None:
+        chroma_updates["enabled"] = overrides["chroma"]
     despill_updates: dict[str, Any] = {}
     if overrides.get("despill") is not None:
         despill_updates["enabled"] = overrides["despill"]
@@ -130,7 +133,7 @@ def override_settings(settings: PipelineSettings, **overrides: Any) -> PipelineS
         else settings.screen_color
     )
     return PipelineSettings(
-        chroma=chroma,
+        chroma=_update_dataclass(chroma, chroma_updates),
         trimap=_update_dataclass(trimap, trimap_updates),
         alpha_post=alpha_post,
         cross_check=_update_dataclass(settings.cross_check, cross_check_updates),