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/config.py

@@ -92,6 +92,8 @@ def override_settings(settings: PipelineSettings, **overrides: Any) -> PipelineS
         ]
         if overrides.get(key) is not None
     }
+    if overrides.get("trimap_mode") is not None:
+        trimap_updates["mode"] = overrides["trimap_mode"]
     despill_updates: dict[str, Any] = {}
     if overrides.get("despill") is not None:
         despill_updates["enabled"] = overrides["despill"]