Fuse semantic segmentation mask into the trimap

Wire BiRefNet into the pipeline (lazy-loaded) and build the trimap from the
semantic subject mask refined by the green key, used when segmentation is
enabled. generate_trimap (chroma-only) stays as the no-segmentation fallback.

Authority split: the mask decides subject topology (keeps green-tinted hair as
foreground, drops see-through holes), the bright-green key sharpens the screen
boundary, ViTMatte refines a small protective band. Background follows the mask
directly so interior holes survive; the band is small so it does not swallow
finger gaps.

End-to-end on the samples: green hair preserved, finger-gap green 99%
transparent (BiRefNet marks it background; was 100% foreground before), leg
edges smooth -- topology/colour/edge problems local cues could not solve, with
no hard alpha surgery.

- SegmentationSettings wired into PipelineSettings/config; --device propagates.
- TrimapSettings: seg_core_threshold, seg_loose_threshold.
- New debug output seg_mask.png.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bgfilter/config.py

@@ -11,6 +11,7 @@ from .settings import (
     ForegroundSettings,
     ModelSettings,
     PipelineSettings,
+    SegmentationSettings,
     TrimapSettings,
 )
 
@@ -42,7 +43,7 @@ def _update_dataclass(instance: T, values: dict[str, Any] | None) -> T:
 
 
 def settings_from_dict(data: dict[str, Any]) -> PipelineSettings:
-    allowed_sections = {"chroma", "trimap", "alpha_post", "foreground", "despill", "model"}
+    allowed_sections = {"chroma", "trimap", "alpha_post", "foreground", "despill", "model", "segmentation"}
     unknown_sections = sorted(set(data) - allowed_sections)
     if unknown_sections:
         raise ValueError(f"Unknown config section(s): {', '.join(unknown_sections)}")
@@ -54,6 +55,7 @@ def settings_from_dict(data: dict[str, Any]) -> PipelineSettings:
         foreground=_update_dataclass(ForegroundSettings(), data.get("foreground")),
         despill=_update_dataclass(DespillSettings(), data.get("despill")),
         model=_update_dataclass(ModelSettings(), data.get("model")),
+        segmentation=_update_dataclass(SegmentationSettings(), data.get("segmentation")),
     )
 
 
@@ -70,6 +72,7 @@ def override_settings(settings: PipelineSettings, **overrides: Any) -> PipelineS
     foreground = settings.foreground
     despill = settings.despill
     model = settings.model
+    segmentation = settings.segmentation
 
     model_updates = {
         key: overrides[key]
@@ -89,6 +92,9 @@ def override_settings(settings: PipelineSettings, **overrides: Any) -> PipelineS
     despill_updates: dict[str, Any] = {}
     if overrides.get("despill") is not None:
         despill_updates["enabled"] = overrides["despill"]
+    seg_updates: dict[str, Any] = {}
+    if overrides.get("device") is not None:
+        seg_updates["device"] = overrides["device"]
 
     return PipelineSettings(
         chroma=chroma,
@@ -97,4 +103,5 @@ def override_settings(settings: PipelineSettings, **overrides: Any) -> PipelineS
         foreground=foreground,
         despill=_update_dataclass(despill, despill_updates),
         model=_update_dataclass(model, model_updates),
+        segmentation=_update_dataclass(segmentation, seg_updates),
     )