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/settings.py

@@ -6,12 +6,12 @@ from dataclasses import dataclass
 @dataclass(frozen=True)
 class ChromaSettings:
     # False = non-flat-background mode (--no-chroma): no colour key at all.
-    # Segmentation alone drives the trimap (mode forced to "seg"), and every
-    # colour-keyed stage is bypassed: background auto-detect, the directional
-    # hue split, chroma alpha suppression, despill, the cross-check veto (its
-    # suspect zone is background-hued by definition; with reuse_as_seg the
-    # second opinion still serves as the segmenter), and the unmix foreground
-    # fallback. Requires segmentation.enabled and matting_method 'vitmatte'.
+    # Segmentation alone drives the trimap (mode forced to "seg"), and the
+    # colour-keyed stages are bypassed: background auto-detect, the directional
+    # hue split, chroma alpha suppression, despill, and the unmix foreground
+    # fallback. The cross-check veto still runs (if enabled) with a hue-free
+    # gate -- see CrossCheckSettings.second_lo/hi. Requires
+    # segmentation.enabled and matting_method 'vitmatte'.
     enabled: bool = True
     border_ratio: float = 0.04
     min_samples: int = 2048
@@ -107,6 +107,13 @@ class CrossCheckSettings:
     feather_sigma: float = 2.0  # Gaussian feather of the zone boundary, in px
     gate_lo: float = 0.70       # primary alpha below this -> fully exempt
     gate_hi: float = 0.95       # primary alpha above this -> fully vetoable
+    # Complex-background mode only (chroma disabled -> no hue-defined suspect
+    # zone): the veto instead requires the second opinion itself to be
+    # confidently near-empty -- full strength at/below second_lo, none at/above
+    # second_hi. Thin structures the downsampled second model merely blurs to
+    # mid-alpha stay untouched; only decisively-rejected regions can be cleared.
+    second_lo: float = 0.15
+    second_hi: float = 0.40
 
 
 @dataclass(frozen=True)