Align defaults with server-edition: reuse-as-seg and despill both OFF

Adopt server-edition's settings.py and default.yaml verbatim (zero diff on
these files between branches now):

- reuse_as_seg: false. The reuse equivalence holds for solid figures but
  breaks on thin pale strokes over a low-contrast background: HR-matting
  (trained on P3M-10k portraits + AM-2k animals) reads them as
  semi-transparent wisps, and as the trimap topology source it destroys
  white calligraphy (TestImage4: stroke alpha 0.92 -> 0.51, 48% of stroke
  pixels below 0.5) that the DIS5K-trained segmenter keeps solid (seg 0.97
  vs 0.40). As the gated cross-check veto the same signal stays harmless
  (hue gate excludes neutral strokes; confidence gate exempts soft edges).
  Opt in with --cross-check-as-seg for character batches.

- despill.enabled: false (mirrors server ba5d667): no positional/semantic
  guard, desaturates and hue-shifts subjects sharing the background's hue.

README updated accordingly; the performance table notes its numbers were
measured with reuse on.

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

bgfilter/cli.py

@@ -31,7 +31,7 @@ def main(
     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"),
     cross_check: bool | None = typer.Option(None, "--cross-check/--no-cross-check", help="Second-opinion veto of background-hued residue between hair strands (default: on; costs one extra model inference)"),
-    cross_check_as_seg: bool | None = typer.Option(None, "--cross-check-as-seg/--no-cross-check-as-seg", help="Reuse the cross-check forward as the segmentation mask, skipping the primary seg model (default: on; saves ~20s on CPU; needs cross-check on)"),
+    cross_check_as_seg: bool | None = typer.Option(None, "--cross-check-as-seg/--no-cross-check-as-seg", help="Reuse the cross-check forward as the segmentation mask, skipping the primary seg model (default: off; saves ~20s on CPU; needs cross-check on)"),
     trimap_mode: str | None = typer.Option(None, "--trimap-mode", help="Trimap mode (segmentation pipeline): directional | seg | directional-hard-bg"),
     seg_backend: str | None = typer.Option(None, "--seg-backend", help="Segmentation backend: birefnet (default) | anime-seg"),
 ) -> None: