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>

README.md

@@ -38,6 +38,13 @@ download (~0.9 GB) and one inference pass per image; disable with
 `--no-cross-check` (see the `cross_check` config section, and
 `docs/hair_gap_artifacts.md` for the analysis behind it).
 
+When the cross-check is on, that same HR-matting forward is **reused as the
+segmentation mask by default**, skipping the primary seg model entirely (one
+less model to load, ~20 s faster per image on CPU). Pilot-validated
+(TestImage3 / FixImage1): trimap 99.8% identical, no structural change to
+fingers, hair wisps or thin lines. Disable with `--no-cross-check-as-seg` to
+run the dedicated seg model instead.
+
 The segmentation trimap defaults to `directional` mode (chroma + seg + a hue-direction
 split: it keeps a background-coloured garment such as a white shirt while dropping a
 background-hued residual such as blue trapped between hair strands). Switch with
@@ -90,8 +97,9 @@ No `--config` is needed — the built-in defaults are identical to
 `configs/default.yaml`. Pass `--config configs\default.yaml` only after you edit that
 file to tune the detailed parameters. Runtime choices stay on the command line:
 `--device` (default **CPU**; use `--device cuda` for GPU), `--precision` (default
-`fp32`; `bf16` halves the matting model's activation memory with visually identical
-alpha — needs bf16-capable hardware, falls back to fp32 elsewhere), `--seg-backend`
+`fp32`; `bf16` speeds up all three models and halves the matting model's activation
+memory with visually identical alpha — needs bf16-capable hardware, falls back to
+fp32 elsewhere), `--seg-backend`
 (default `birefnet`; `anime-seg` for anime characters), `--screen-color` (default:
 auto-detect the flat background), and `--trimap-mode`.