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>

configs/default.yaml

@@ -25,6 +25,15 @@ trimap:
   fg_safe_radius_ratio: 0.006
   seg_core_threshold: 0.60
   seg_loose_threshold: 0.08
+  # Trimap mode (segmentation pipeline only; also settable via --trimap-mode):
+  #   directional          = chroma + seg + hue split; background-hued residual
+  #                          (blue between hair) -> unknown, white shirt kept. (default)
+  #   seg                  = original fuse_trimap (no hue split).
+  #   directional-hard-bg  = hard-remove background-hued pixels (aggressive; can eat
+  #                          cool/shadowed white cloth).
+  mode: directional
+  seg_low: 0.15
+  bg_hue_proj_min: 4.0
 
 alpha_post:
   chroma_suppress: true