Run the cross-check veto in complex-background mode with a hue-free gate

The veto's suspect zone was defined by background-hue projection, so
--no-chroma silently skipped it. Without a key colour there is no hue
test to lean on, so in complex mode the gate instead requires the second
opinion itself to be confidently near-empty (second_lo -> second_hi ramp,
default 0.15 -> 0.40): regions the HR-matting model decisively rejects
can be cleared, while thin strands it merely blurs to mid-alpha are
untouched -- protecting exactly the crisp-strand advantage the pipeline
has over a raw BiRefNet mask. Flat mode's gate is unchanged.

Validation (BG_IMAGE01-04, cuda fp32): flat default path bit-identical;
complex-mode veto touches 0.007-0.025% of pixels, visibly clearing
blurred residue near strands and milky specks in hair gaps with no
strand erosion. Costs the HR-matting forward in --no-chroma runs
(0.61 -> 1.6 s/image warm GPU); disable with --no-cross-check.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bgfilter/pipeline.py

@@ -199,16 +199,16 @@ def _process_rgb(rgb: np.ndarray, pipeline: MattingPipeline) -> MattingResult:
             raw_alpha=raw_alpha if alpha_source == "vitmatte" else None,
         )
     alpha = clean_alpha(alpha, trimap, settings.alpha_post)
-    # The veto's suspect zone is defined by background hue, so it needs the key
-    # colour; without chroma the second opinion still serves as seg (reuse above).
-    if settings.cross_check.enabled and settings.chroma.enabled:
+    if settings.cross_check.enabled:
         lab = convert_color_spaces(rgb)[1]
         if second_alpha is None:
             second_alpha = pipeline._second_opinion(rgb)
         alpha = cross_check_alpha(
             alpha,
             second_alpha,
-            bg_hue_projection(lab, model.lab_center),
+            # No key colour in complex-background mode: proj=None switches the
+            # veto to its second-opinion-confidence gate (see cross_check_alpha).
+            bg_hue_projection(lab, model.lab_center) if model is not None else None,
             lab[..., 0],
             trimap,
             settings.cross_check,