Auto-detect the background colour (default); drop the green heuristic

Replace the hardcoded green-screen auto-detection with a general flat-colour
detector. When screen_color is null the background colour is now detected from the
image border (dominant colour of the border strip) and used directly as the chroma
model, with a failure gate that raises when the border is not one clean flat colour
(gradient / texture / subject filling the frame).

- chroma: add detect_background_color + _background_border_cluster; the null case of
  estimate_background_model samples the detected cluster directly (no seed search);
  compute_bg_confidence is now always perceptual (Lab/RGB distance). Removed the green
  heuristic (initial_green_candidates), the green multiplicative gating, the now-unused
  _smoothstep, and five green-only ChromaSettings fields.
- pipeline: reuse the detected colour for de-spill in the auto case.
- settings / default.yaml: add detect_* tuning fields; refresh screen_color docs.
- README: document auto-detection + the failure gate, trimap modes, birefnet config.

A supplied --screen-color hex still uses the seed-search refinement. Validated:
detects green / pastel / text samples correctly, raises on gradient / two-colour, CLI
exits 1 on failure, green and pastel mattes unchanged in quality.

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

bgfilter/cli.py

@@ -8,7 +8,7 @@ from rich import print
 from .config import load_settings, override_settings
 from .pipeline import MattingPipeline, run_image
 
-app = typer.Typer(help="Offline green screen character matting.")
+app = typer.Typer(help="Offline flat-background character matting.")
 
 
 @app.command()
@@ -21,7 +21,7 @@ def main(
     config: Path | None = typer.Option(None, "--config", exists=True, file_okay=True, dir_okay=False),
     model_name: str | None = typer.Option(None, "--model-name"),
     device: str | None = typer.Option(None, "--device"),
-    screen_color: str | None = typer.Option(None, "--screen-color", help="Background colour prior as #RRGGBB (default: auto-detect green screen)"),
+    screen_color: str | None = typer.Option(None, "--screen-color", help="Background colour prior as #RRGGBB (default: auto-detect the flat background colour)"),
     matting_method: str | None = typer.Option(None, "--matting-method"),
     fallback_to_chroma_alpha: bool | None = typer.Option(None, "--fallback-to-chroma-alpha/--no-fallback-to-chroma-alpha"),
     sure_bg_threshold: float | None = typer.Option(None, "--sure-bg-threshold", min=0.0, max=1.0),