Default reuse-as-seg OFF; default expandable_segments on Linux/WSL

Two default flips for server-edition:

1. cross_check.reuse_as_seg now defaults to false (settings + default.yaml + CLI help): the dedicated segmenter keeps its own forward and the cross-check veto stays an independent second signal. The reuse remains available via --cross-check-as-seg / config; measured cost of off vs on: ~20 s/image on CPU, +0.1-0.25 s and +0.9-1.3 GB VRAM on GPU (13.3 vs 12.0 GB reserved under expandable segments -- still fits a 16 GB card).

2. bgfilter/__init__.py defaults PYTORCH_CUDA_ALLOC_CONF=expandable_segments:True on non-Windows platforms, before torch loads (setdefault: an explicit env value wins; native Windows is excluded because torch warns and ignores it there). Measured: reserved 15.3 -> 12.0 GB and ~10% faster on the RTX 5070 Ti; no effect on CPU-only runs.

README/DEPLOY(_ZH) synced: reuse documented as opt-in, GPU profile numbers updated for both reuse states, perf table labeled with the config it was measured under.

Verified: defaults resolve off/set as intended on Windows and WSL; explicit PYTORCH_CUDA_ALLOC_CONF override wins.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bgfilter/settings.py

@@ -76,12 +76,13 @@ class CrossCheckSettings:
     input_size: int = 2048
     # Reuse the cross-check forward as the segmentation mask: one HR-matting
     # 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
-    # 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
+    # primary seg model entirely (saves its load + forward; measured ~20 s/image
+    # on CPU, ~0.9-1.3 GB VRAM on GPU). Pilot-validated on TestImage3/FixImage1:
+    # trimap 99.8% identical. Default OFF on server-edition: the dedicated
+    # segmenter keeps its own forward and the veto stays an independent signal;
+    # opt in with --cross-check-as-seg. Only applies to the birefnet backend
+    # (a same-family swap); needs cross_check and segmentation both enabled.
+    reuse_as_seg: bool = False
     # Compute precision of the cross-check forward: "fp32" or "bf16" (autocast;
     # needs bf16-capable hardware, else falls back to fp32 with a warning).
     precision: str = "fp32"