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/pipeline.py

@@ -111,7 +111,16 @@ def _run_image(
     )
     second_alpha = None
     if settings.segmentation.enabled:
-        if settings.cross_check.enabled and settings.cross_check.reuse_as_seg:
+        reuse = (
+            settings.cross_check.enabled
+            and settings.cross_check.reuse_as_seg
+            # Reuse swaps one BiRefNet-family model for another (validated
+            # equivalent topology). A different backend chosen explicitly
+            # (anime-seg) keeps its own forward; cross-check still runs
+            # independently on top of it.
+            and settings.segmentation.backend == "birefnet"
+        )
+        if reuse:
             # One HR-matting forward serves both the trimap topology and the
             # cross-check second opinion; the primary seg model is never loaded.
             second_alpha = pipeline._second_opinion(rgb)