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/pipeline.py

@@ -56,6 +56,7 @@ class MattingPipeline:
                     model_name=check.model_name,
                     device=self.settings.segmentation.device,
                     input_size=check.input_size,
+                    precision=check.precision,
                 )
             )
         return self._cross_checker.mask(rgb)
@@ -108,8 +109,15 @@ def _run_image(
     bg_confidence, model = compute_bg_confidence(
         rgb, settings=settings.chroma, screen_color=screen_color
     )
+    second_alpha = None
     if settings.segmentation.enabled:
-        seg_mask = pipeline._segment(rgb)
+        if settings.cross_check.enabled and settings.cross_check.reuse_as_seg:
+            # 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)
+            seg_mask = second_alpha
+        else:
+            seg_mask = pipeline._segment(rgb)
         mode = settings.trimap.mode
         if mode == "seg":
             trimap, trimap_stats = fuse_trimap(seg_mask, bg_confidence, settings.trimap)
@@ -136,10 +144,10 @@ def _run_image(
         raw_alpha=raw_alpha if alpha_source == "vitmatte" else None,
     )
     alpha = clean_alpha(alpha, trimap, settings.alpha_post)
-    second_alpha = None
     if settings.cross_check.enabled:
         lab = convert_color_spaces(rgb)[1]
-        second_alpha = pipeline._second_opinion(rgb)
+        if second_alpha is None:
+            second_alpha = pipeline._second_opinion(rgb)
         alpha = cross_check_alpha(
             alpha,
             second_alpha,