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>

README.md

@@ -43,7 +43,10 @@ segmentation mask by default**, skipping the primary seg model entirely (one
 less model to load, ~20 s faster per image on CPU). Pilot-validated
 (TestImage3 / FixImage1): trimap 99.8% identical, no structural change to
 fingers, hair wisps or thin lines. Disable with `--no-cross-check-as-seg` to
-run the dedicated seg model instead.
+run the dedicated seg model instead. The reuse is a same-family swap, so it
+applies to the `birefnet` backend only: with `--seg-backend anime-seg` the
+anime segmenter keeps its own forward and the cross-check veto still runs
+independently on top.
 
 The segmentation trimap defaults to `directional` mode (chroma + seg + a hue-direction
 split: it keeps a background-coloured garment such as a white shirt while dropping a