Add chroma-guided alpha suppression for background holes

Pull alpha toward 0 in the unknown band where bg_confidence is high, so green
that survives in hair gaps and hole pockets goes transparent (doc section 10).
Runs between trimap enforcement and alpha cleanup; sure-foreground pixels are
never touched, keyed by the bg_confidence we already compute.

On the two samples this clears the faint outer green halo (edge pixels
19.3k -> 16.0k) but has little *visible* effect, because their residual is no
longer green: it is magenta from the pymatting unmix on genuine semi-transparent
hair (vis-magenta p95 ~0.07, identical pre/post despill), which this pass
deliberately leaves alone. The magenta needs a separate colour fix.

Knobs live in AlphaPostSettings (chroma_suppress*, default on).

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

bgfilter/settings.py

@@ -33,6 +33,12 @@ class AlphaPostSettings:
     fill_hole_area_ratio: float = 0.00002
     alpha_floor: float = 0.002
     alpha_ceil: float = 0.998
+    # Chroma-guided alpha suppression: in the unknown band, pull alpha toward 0
+    # where background (green) confidence is high, clearing spill in gaps/holes.
+    chroma_suppress: bool = True
+    chroma_suppress_bg_low: float = 0.35
+    chroma_suppress_bg_high: float = 0.80
+    chroma_suppress_strength: float = 1.0
 
 
 @dataclass(frozen=True)