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/service.py

@@ -105,6 +105,7 @@ class PipelineManager:
                             model_name=check.model_name,
                             device=self._base.segmentation.device,
                             input_size=check.input_size,
+                            precision=check.precision,
                         )
                     )
         return self._cross_checker