Add configs/cpu-fast.yaml: fp32 CPU lane, no cross-check, no anime-seg

Urgent lightweight production profile for a CPU-only fp32 server: base ViTMatte + one BiRefNet@1024, cross-check disabled. Provisions only those two weights (no BiRefNet_HR-matting, no anime-seg) and returns to the ~14-15s/image baseline. Run with BGFILTER_CONFIG=configs/cpu-fast.yaml.

Verified end-to-end: loads exactly 2 models, cross_check resolves off, service preloads the primary segmenter and not the cross-checker.

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

configs/cpu-fast.yaml

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+# Urgent CPU-only lane: fp32, birefnet segmentation, NO cross-check, NO anime-seg.
+#
+# This is the lightest production path: base ViTMatte + one BiRefNet@1024, ~14-15s
+# per 1.5 MP image on CPU. It needs ONLY these two weights provisioned --
+# BiRefNet_HR-matting (cross-check) and anime-seg are never loaded.
+#
+# Anything omitted here inherits the built-in defaults (identical to default.yaml).
+
+model:
+  device: cpu
+  precision: fp32          # fp32 only: this lane targets non-bf16 CPUs
+
+segmentation:
+  enabled: true
+  backend: birefnet        # the only backend this lane uses
+  device: cpu
+  precision: fp32
+
+cross_check:
+  enabled: false           # no second-opinion HR-matting pass (the big cost/mem sink)