Add seg_force_fg: saturated seg confidence overrides the hue-split demotion

New rule 4 in fuse_trimap_directional: a chroma-unknown pixel whose seg
confidence is >= trimap.seg_force_fg (default 0.98) stays sure-FG even when
background-hued. Bright skin on a warm same-hue-family background (peach,
pale yellow) was being demoted to unknown by the hue split and then lost --
either ViTMatte itself misfires there (raw 0.07-0.13 measured) or the
chroma suppressor does (raw 0.89 halved to 0.45 via the RGB-proximity
confidence path). The segmenter meanwhile rates those pixels a saturated
1.0; that semantic certainty now outranks same-hue colour suspicion.

Safety: chroma_unknown excludes sure background, so a real flat backdrop
can never be forced foreground; the silhouette band still re-opens the
boundary; the cross-check veto still overrides trimap-FG. Set > 1.0 to
disable.

Verified (GPU bf16): yellow-bg frame face 0.84->1.00, peach-bg thigh
0.87->1.00 (residual softness only where chroma is near-sure-bg, by
design). Pastel-blue TestImage3 regression: 0.047% of pixels differ with
cross-check on, 0.058% without it (hair-gap suppression path), both
visually negligible.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

configs/default.yaml

@@ -46,6 +46,12 @@ trimap:
   mode: directional
   seg_low: 0.15
   bg_hue_proj_min: 4.0
+  # Semantic override (directional mode): chroma-unknown pixels with seg confidence
+  # at/above this stay sure-FG even when background-hued — rescues bright skin on a
+  # warm (same-hue-family) background that the hue split would otherwise hand to
+  # ViTMatte/suppression, which misfire exactly there. Never overrides chroma
+  # sure-background. Set > 1.0 to disable.
+  seg_force_fg: 0.98
 
 alpha_post:
   chroma_suppress: true