Disable despill by default (all lanes, incl. cpu-fast)

despill pulls foreground chroma along the background-hue axis with no
positional or semantic guard, so a subject sharing the background's hue
(e.g. a blue suit on a blue backdrop) is desaturated and hue-shifted
(measured ~ΔE 35, blue -> purple) — a false positive on correct colours.

Flip the DespillSettings.enabled dataclass default to False so every lane
that omits a despill section — notably cpu-fast — inherits it off, and
mirror it in default.yaml. Re-enable per-config or with --despill when
edge spill genuinely matters.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

configs/default.yaml

@@ -110,7 +110,10 @@ foreground:
   bg_confidence_weight: 0.70
 
 despill:
-  enabled: true
+  # Off: despill has no positional/semantic guard, so a subject sharing the
+  # background's hue (blue suit on a blue backdrop) gets desaturated/hue-shifted
+  # (~ΔE 35, blue -> purple). Re-enable per-job only when edge spill matters.
+  enabled: false
   edge_low: 0.005
   strength: 0.92
   edge_expand_radius: 2