Merge origin/master: bf16 precision, chunked attention, seg-reuse, mimalloc purge

Adopts master as the standard for all overlapping work. Master's landed
optimizations supersede server-edition's own fp16 experiment:

- Unified --precision fp32|bf16 knob (bgfilter/precision.py) driving all
  three models: ViTMatte weight cast + both BiRefNets via autocast, with a
  hardware gate (falls back to fp32 off native-bf16 CPUs) and an
  AutocastCPU fp32 shim for torchvision deform_conv2d.
- Query-chunked ViTMatte global attention (bgfilter/attn_chunk.py), exact
  and bitwise-identical, caps the N^2 spike (~19 -> ~4 GB at 2048).
- Cross-check HR-matting forward reused as the seg mask (birefnet backend
  only), skipping the primary seg model; MIMALLOC_PURGE_DELAY=0.
- inference_mode and the detect_background_color removal converge with
  server-edition's earlier equivalents.

Conflict resolution (favoring master, preserving server-only features):
- vitmatte_infer/segmentation: dropped server's device-derived fp16 for
  master's precision path, kept resolve_model_source (local weights).
- service.py: cross-check SegmentationSettings now passes precision so the
  HTTP service honors bf16 like the CLI's _second_opinion does.

Verified on CPU: default fp32 pipeline loads 2 models (seg-reuse active)
and bf16 path runs (autocast + deform_conv2d shim) — both exit 0.

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

bgfilter/pipeline.py

@@ -85,6 +85,7 @@ 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)
@@ -135,8 +136,24 @@ def _process_rgb(rgb: np.ndarray, pipeline: MattingPipeline) -> MattingResult:
     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)
+        reuse = (
+            settings.cross_check.enabled
+            and settings.cross_check.reuse_as_seg
+            # Reuse swaps one BiRefNet-family model for another (validated
+            # equivalent topology). A different backend chosen explicitly
+            # (anime-seg) keeps its own forward; cross-check still runs
+            # independently on top of it.
+            and settings.segmentation.backend == "birefnet"
+        )
+        if reuse:
+            # 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)
@@ -163,10 +180,10 @@ def _process_rgb(rgb: np.ndarray, pipeline: MattingPipeline) -> MattingResult:
         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,