Gate chroma suppression by the matte's own confidence

suppress_alpha_by_chroma was the one stage where colour evidence could veto the
matting model outright, and every observed failure had the same shape: ViTMatte
was right (raw ~0.97 on a white shirt over pastel blue, on blush over pink) and
the bgc-keyed suppression crushed it, because bgc magnitude cannot tell a
background-coloured subject from background residue. The matte's raw alpha can:
it rates enclosed background pockets low (~0.28 median) and subject texture high.

Suppression is now scaled by (1 - smoothstep(raw_alpha, suppress_raw_lo,
suppress_raw_hi)) (defaults 0.85/0.98): colour evidence only vetoes the matte
where the matte itself is unsure. The gate applies only when alpha really comes
from ViTMatte; chroma-seeded alpha passes raw_alpha=None and keeps old behaviour.
Set suppress_raw_lo: 1.0 to disable (verified byte-identical to the old output).

Validated on both screen colours: pink #F4D8E8 face restored (skin alpha 0.841 ->
0.993, holes 17.2% -> 0.6%), pastel blue #CFEFFF unchanged-to-better (shirt +0.01,
wisps identical, gap clearing 99.8%). Also fixes the long-deferred chroma-only
pipeline defects (white shirt 0.900, light skin 0.989). CLI output verified
byte-identical to the reviewed prototype on both images.

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

configs/default.yaml

@@ -40,6 +40,11 @@ alpha_post:
   chroma_suppress_bg_low: 0.35
   chroma_suppress_bg_high: 0.80
   chroma_suppress_strength: 1.0
+  # Matte-confidence gate: suppression fades where ViTMatte's raw alpha is high
+  # (ramp lo -> hi), so colour evidence only vetoes the matte where it is unsure.
+  # Set suppress_raw_lo: 1.0 to disable.
+  suppress_raw_lo: 0.85
+  suppress_raw_hi: 0.98
 
 foreground:
   enabled: true