Use pymatting ML foreground estimation to remove edge spill

After the full-foreground despill (5ddf0cd) the residual green was confined to
semi-transparent hair (alpha < 0.5): 85% of the worst pixels had alpha < 0.3,
where a pixel-wise unmix (divide by small alpha) is too noisy to trust.

Replace the hand-rolled unmix/local-blur estimator with pymatting's
estimate_foreground_ml (Germer et al. multi-level closed form), which propagates
reliable foreground colour from high-alpha neighbours into the fringe and
estimates the background, so green spill is unmixed rather than clamped. The
despill pass stays as a light cleanup on top.

- foreground.method selects "ml" (default) or "unmix" (legacy heuristic kept as
  a fallback when pymatting is unavailable, matching the chroma fallback idiom).
- ForegroundEstimate now exposes rgb (F), background (B) and a correction map;
  debug outputs become foreground_rgb / foreground_background / foreground_correction.

Controlled comparison (same ViTMatte alpha, only foreground method changed),
edge_green_excess:
  TestImage  p95 0.176->0.031, mean 0.061->0.005
  TestImage2 p95 0.165->0.004, mean 0.052->0.002

Adds pymatting (pulls in numba) to requirements.

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

bgfilter/pipeline.py

@@ -95,10 +95,9 @@ def _run_image(
         save_gray(debug / "bg_confidence.png", bg_confidence)
         save_gray(debug / "trimap.png", trimap)
         save_gray(debug / "alpha.png", alpha)
-        save_gray(debug / "foreground_mask.png", foreground.mask)
-        save_gray(debug / "foreground_unmix_mask.png", foreground.unmix_mask)
-        save_gray(debug / "foreground_local_mask.png", foreground.local_mask)
         save_rgb(debug / "foreground_rgb.png", foreground.rgb)
+        save_rgb(debug / "foreground_background.png", foreground.background)
+        save_gray(debug / "foreground_correction.png", foreground.correction)
         save_gray(debug / "despill_mask.png", despill_mask)
         save_previews(debug, corrected_rgb, alpha)
         tiles = {