Align default ViTMatte model to base; make --config optional

The ModelSettings.model_name dataclass default was still vitmatte-small (the
phase-1 "start small" choice) while configs/default.yaml used vitmatte-base, so a
bare run (no --config) silently differed from every documented example. Set the
dataclass default to base too: the built-in defaults now match default.yaml
field-for-field, so --config is only needed after editing that file to tune
parameters. README examples drop the now-redundant --config.

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

README.md

@@ -73,16 +73,15 @@ D:\MiniConda\envs\lightML\python.exe -m bgfilter.cli --help
 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
+  --debug-dir Outputs\TestImage_debug
 ```
 
-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`, `--seg-backend`) stay on the command line. The default device is
-**CPU**; add `--device cuda` for GPU.
+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), `--seg-backend` (default
+`birefnet`; `anime-seg` for anime characters), `--screen-color` (default: auto-detect
+the flat background), and `--trimap-mode`.
 
 ## Batch
 
@@ -90,8 +89,7 @@ usually happens in YAML while runtime choices (`--device`, `--screen-color`,
 D:\MiniConda\envs\lightML\python.exe -m bgfilter.cli `
   --input-dir Samples `
   --output-dir Outputs `
-  --debug-dir Outputs\debug `
-  --config configs\default.yaml
+  --debug-dir Outputs\debug
 ```
 
 ## Chroma-alpha debug mode

bgfilter/settings.py

@@ -90,7 +90,7 @@ class DespillSettings:
 
 @dataclass(frozen=True)
 class ModelSettings:
-    model_name: str = "hustvl/vitmatte-small-composition-1k"
+    model_name: str = "hustvl/vitmatte-base-composition-1k"
     device: str = "cpu"
     matting_method: str = "vitmatte"
     fallback_to_chroma_alpha: bool = False