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>

README.md

@@ -84,10 +84,9 @@ For each processed image, the CLI writes an RGBA PNG and optional debug files:
 bg_confidence.png
 trimap.png
 alpha.png
-foreground_mask.png
-foreground_unmix_mask.png
-foreground_local_mask.png
 foreground_rgb.png
+foreground_background.png
+foreground_correction.png
 despill_mask.png
 preview_black.png
 preview_white.png
@@ -125,5 +124,7 @@ D:\MiniConda\envs\lightML\python.exe scripts\smoke_samples.py `
 
 - `Samples/` and `Outputs/` are ignored by Git.
 - `ViTMatte` model weights are loaded from Hugging Face on first use.
-- Foreground color estimation runs before final de-spill and writes debug masks
-  for unmixing and local foreground propagation.
+- Foreground color estimation uses pymatting's `estimate_foreground_ml` to
+  propagate clean foreground colour into semi-transparent edges before final
+  de-spill, writing the estimated foreground, background, and a correction map.
+  Set `foreground.method: unmix` to fall back to the legacy heuristic.