Defined in Task 2 as a palette slot for section-disabled header styling,
but the final code path never used it — disabled sections rely on egui's
add_enabled auto-dimming instead of a manual header color. Removes the
last dead-code warning from the UI rewrite.
Plan transcribed #3C4646 as rgb(60,70,90)=#3C465A (20 off on blue);
spec §1 specifies rgb(60,70,70). Also add module-level egui-0.35
version-drift note and hex comments on inline widget-state colors
for grep-able spec↔code comparison.
Palette constants + sci_fi_visuals() + setup(). Hand-applied over
Visuals::dark(); no custom font, no new deps. Not yet wired into
ui_system (Task 4 does that via setup_egui_style).
