Rescue ViTMatte dropouts with the segmentation mask

Add fill_seg_dropouts: where ViTMatte's alpha is ~0 but BiRefNet confidently
asserts foreground, restore a faint alpha from the mask (strength * seg_mask).
This recovers green-contaminated wisps that ViTMatte cuts because they are
nearly the background colour, while leaving ViTMatte's sharper edges elsewhere
untouched (gated to true dropouts) and not refilling the finger gap (low mask).

On the TestImage2 shoulder, faint wisps ViTMatte cut drop from 72% to 33%; the
finger gap stays transparent. The rescued wisps are still green-tinted -- the
recolor pass (next) restores their colour.

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

configs/default.yaml

@@ -23,6 +23,10 @@ alpha_post:
   chroma_suppress_bg_low: 0.35
   chroma_suppress_bg_high: 0.80
   chroma_suppress_strength: 1.0
+  seg_fill: true
+  seg_fill_alpha_max: 0.10
+  seg_fill_seg_min: 0.10
+  seg_fill_strength: 0.70
 
 foreground:
   enabled: true