Add directional trimap mode (default); keep seg/hard-bg via trimap.mode

Replace the two experimental bools (directional, bg_hued_to_bg) with a single
TrimapSettings.mode selector for the segmentation pipeline:
  - "directional" (new default): chroma magnitude + seg + a Lab hue-direction
    split. In the chroma-unknown zone a confidently-segmented pixel stays
    foreground unless it is displaced toward the background hue, so a neutral
    background-coloured garment (e.g. a white shirt) is kept while a background-
    hued residual (blue between hair strands) is left unknown for ViTMatte /
    chroma-suppress to clear.
  - "seg": original fuse_trimap, unchanged.
  - "directional-hard-bg": aggressive variant that hard-removes background-hued
    pixels (can eat cool/shadowed white cloth).

Selectable via configs/default.yaml (trimap.mode) or CLI --trimap-mode; unknown
modes raise. Default CLI output verified byte-identical to the reviewed UNK
result on the pastel sample.

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

bgfilter/cli.py

@@ -29,6 +29,7 @@ def main(
     unknown_radius_ratio: float | None = typer.Option(None, "--unknown-radius-ratio", min=0.0),
     fg_safe_radius_ratio: float | None = typer.Option(None, "--fg-safe-radius-ratio", min=0.0),
     despill: bool | None = typer.Option(None, "--despill/--no-despill"),
+    trimap_mode: str | None = typer.Option(None, "--trimap-mode", help="Trimap mode (segmentation pipeline): directional | seg | directional-hard-bg"),
 ) -> None:
     try:
         settings = override_settings(
@@ -43,6 +44,7 @@ def main(
             unknown_radius_ratio=unknown_radius_ratio,
             fg_safe_radius_ratio=fg_safe_radius_ratio,
             despill=despill,
+            trimap_mode=trimap_mode,
         )
 
         if input_dir is not None: