Cut pipeline cost: seg-reuse, unified bf16 knob, eager mimalloc purge

Three optimizations from profiling the cross-check-dominated pipeline
(9700X CPU, all pilot-validated on TestImage3/FixImage1):

- Reuse the cross-check HR-matting@2048 forward as the segmentation mask
  (cross_check.reuse_as_seg, default ON; --no-cross-check-as-seg to opt
  out). Skips the BiRefNet@1024 load+forward entirely: ~66s -> ~45s,
  one less 0.9GB model. Trimap 99.8% identical, no structural change.

- --precision bf16 now fans out to all three models: ViTMatte keeps its
  weight cast; both BiRefNets run their forward under autocast with a
  dispatcher-level AutocastCPU fp32 shim for torchvision::deform_conv2d
  (no bf16 CPU kernel, no autocast wrapper upstream). Shared hardware
  gate in bgfilter/precision.py falls back to fp32 off native-bf16
  hardware. TestImage3: 51.9s -> 33.1s; alpha diff max 0.15, none >0.25.

- MIMALLOC_PURGE_DELAY=0 (bgfilter/__init__.py, before torch loads):
  Windows torch's bundled mimalloc lazily retains ~10GB of freed
  BiRefNet activations, stacking under ViTMatte's attention peak.
  2048x2048 bf16: peak 25.2 -> 21.4GB and slightly faster (73 -> 63s).

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

bgfilter/config.py

@@ -105,9 +105,16 @@ def override_settings(settings: PipelineSettings, **overrides: Any) -> PipelineS
     cross_check_updates: dict[str, Any] = {}
     if overrides.get("cross_check") is not None:
         cross_check_updates["enabled"] = overrides["cross_check"]
+    if overrides.get("cross_check_as_seg") is not None:
+        cross_check_updates["reuse_as_seg"] = overrides["cross_check_as_seg"]
     seg_updates: dict[str, Any] = {}
     if overrides.get("device") is not None:
         seg_updates["device"] = overrides["device"]
+    # --precision is a single knob for all three models (ViTMatte handled via
+    # model_updates above; per-model values remain settable in the config file).
+    if overrides.get("precision") is not None:
+        seg_updates["precision"] = overrides["precision"]
+        cross_check_updates["precision"] = overrides["precision"]
     if overrides.get("seg_backend") is not None:
         backend = overrides["seg_backend"]
         if backend not in _SEG_MODELS: