Gate seg-reuse to the birefnet backend

reuse_as_seg swaps one BiRefNet-family model for another (the validated
equivalence); it must not silently override an explicitly chosen anime-seg
backend. With --seg-backend anime-seg the segmenter now keeps its own
forward and the cross-check veto runs independently on top -- both
combinable, no flag juggling. Default birefnet path is untouched
(verified byte-identical); anime-seg + cross-check verified to run both
models.

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

bgfilter/settings.py

@@ -78,7 +78,9 @@ class CrossCheckSettings:
     # inference serves both trimap topology and the second opinion, skipping the
     # primary seg model entirely (saves its load + forward). Pilot-validated on
     # TestImage3/FixImage1: trimap 99.8% identical, no structural change. Only
-    # takes effect when both cross_check and segmentation are enabled.
+    # applies to the birefnet segmentation backend (a same-family swap); with
+    # anime-seg the chosen segmenter keeps its own forward and the cross-check
+    # runs independently. Needs cross_check and segmentation both enabled.
     reuse_as_seg: bool = True
     # Compute precision of the cross-check forward: "fp32" or "bf16" (autocast;
     # needs bf16-capable hardware, else falls back to fp32 with a warning).