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/pipeline.py

@@ -6,7 +6,7 @@ from pathlib import Path
 
 import numpy as np
 
-from .alpha_post import clean_alpha, enforce_trimap
+from .alpha_post import clean_alpha, enforce_trimap, suppress_alpha_by_chroma
 from .chroma import compute_bg_confidence
 from .despill import despill_green
 from .foreground import estimate_foreground_rgb
@@ -71,6 +71,7 @@ def _run_image(
     alpha, alpha_source = pipeline._predict_alpha(rgb, trimap, bg_confidence)
 
     alpha = enforce_trimap(alpha, trimap)
+    alpha = suppress_alpha_by_chroma(alpha, bg_confidence, trimap, settings.alpha_post)
     alpha = clean_alpha(alpha, trimap, settings.alpha_post)
     foreground = estimate_foreground_rgb(
         rgb, alpha, bg_confidence, model, settings.foreground