Default to BiRefNet + CPU; add --seg-backend to pick the segmenter

- Segmentation backend now defaults to birefnet (ZhengPeng7/BiRefNet); anime-seg is
  selected at runtime with --seg-backend anime-seg, which also swaps in the matching
  weights (unknown backends raise). Config files stay for detailed tuning, not
  backend selection.
- Default device is now cpu everywhere (dataclass defaults, default.yaml,
  birefnet.yaml); pass --device cuda for GPU. Avoids a hard CUDA-required failure on
  machines without a CUDA-enabled PyTorch build.
- Docs: README + workflow status note updated (backend via --seg-backend, CPU
  default, examples no longer force --device cuda).

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

README.md

@@ -20,9 +20,9 @@ Two pipelines, selected by `segmentation.enabled` in the config:
   Lightest / fastest; leans entirely on the colour key for topology.
 - **Single segmenter** (`enabled: true`, default) — one segmentation model drives the
   trimap topology (holes, hair), ViTMatte then refines the soft edges. Backend is
-  switchable: `anime-seg` (default, ONNX, for anime characters) or `birefnet` (general
-  salient objects — text, logos, photos; needs `trust_remote_code`). Use
-  `--config configs\birefnet.yaml` for the BiRefNet backend.
+  switchable: `birefnet` (default, general salient objects — text, logos, photos; needs
+  `trust_remote_code`) or `anime-seg` (ONNX, tuned for anime characters). Switch at
+  runtime with `--seg-backend anime-seg` (it also selects the matching weights).
 
 Both share pymatting foreground estimation and a colour de-spill. There is **no**
 green-contamination rescue / recolour layer — with clean source images it is
@@ -74,15 +74,15 @@ D:\MiniConda\envs\lightML\python.exe -m bgfilter.cli `
   --input Samples\TestImage.png `
   --output Outputs\TestImage_rgba.png `
   --debug-dir Outputs\TestImage_debug `
-  --config configs\default.yaml `
-  --device cuda
+  --config configs\default.yaml
 ```
 
 No `--screen-color` is needed — the background colour is auto-detected. Add
 `--screen-color "#CFEFFF"` only to override it. The CLI reads `configs/default.yaml`
 when `--config` is provided; command-line options override config values, so tuning
 usually happens in YAML while runtime choices (`--device`, `--screen-color`,
-`--trimap-mode`) stay on the command line.
+`--trimap-mode`, `--seg-backend`) stay on the command line. The default device is
+**CPU**; add `--device cuda` for GPU.
 
 ## Batch
 
@@ -91,8 +91,7 @@ D:\MiniConda\envs\lightML\python.exe -m bgfilter.cli `
   --input-dir Samples `
   --output-dir Outputs `
   --debug-dir Outputs\debug `
-  --config configs\default.yaml `
-  --device cuda
+  --config configs\default.yaml
 ```
 
 ## Chroma-alpha debug mode