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>

configs/default.yaml

@@ -72,6 +72,8 @@ cross_check:
   # drives both trimap topology and the veto, skipping the primary seg model
   # (saves its ~9s forward + ~0.9 GB load; CPU pipeline ~66s -> ~45s). Pilot:
   # trimap 99.8% identical, no structural change (fingers/wisps/bowstring OK).
+  # Only applies to the birefnet backend (same-family swap); anime-seg keeps
+  # its own forward, with the cross-check running independently on top.
   # Disable with --no-cross-check-as-seg. No effect when cross_check is off.
   reuse_as_seg: true
   # Cross-check forward precision, same semantics as segmentation.precision