Add Nebula — native C settings panel (Display, Appearance, Input, About)

nebula/nebula.c (~600 lines, single-file, native Xlib/Xft):
- Display tab: resolution cycling via xrandr, DPI toggle (96/144),
  brightness control via brightnessctl
- Appearance tab: wallpaper reset/open, shows active GTK/icon theme
- Input tab: key repeat delay + rate steppers (reads live from XKB),
  tap-to-click and natural scrolling toggles via xinput
- About tab: version, component, platform info
- Fixed-size 700×500 window, dark-navy palette, sidebar nav with
  accent mark on selected tab, dirty-state indicator, Apply/Close
- Escape/Q/Enter shortcuts; Enter applies pending changes

Wired up:
- Root Makefile: nebula target, install -Dm755 nebula/nebula
- Right-click desktop menu: "System Settings" → nebula
- ISO: binary installed to /usr/local/bin/nebula, Super+S xbindkeys
  shortcut, file permissions map entry
- .gitignore: nebula/nebula binary

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/docker/entrypoint.sh

@@ -230,6 +230,7 @@ install -m755 "$SRC/wm/orionwm"              "$AIROOTFS/usr/local/bin/orionwm"
 install -m755 "$SRC/deskbar/oriondeskbar"    "$AIROOTFS/usr/local/bin/oriondeskbar"
 install -m755 "$SRC/launcher/orionlauncher"  "$AIROOTFS/usr/local/bin/orionlauncher"
 install -m755 "$SRC/themes/orionwallpaper"      "$AIROOTFS/usr/local/bin/orionwallpaper"
+install -m755 "$SRC/nebula/nebula"              "$AIROOTFS/usr/local/bin/nebula"
 install -m755 "$SRC/installer/orioninstall"        "$AIROOTFS/usr/local/bin/orioninstall"
 # GUI installer (compiled binary; falls back gracefully if not built)
 if [[ -f "$SRC/installer/orioninstall-gui" ]]; then
@@ -335,6 +336,10 @@ cat > "$SKEL/.xbindkeysrc" <<'XBK'
 "orionlauncher"
   Mod1 + F1
 
+# System settings (Nebula)
+"nebula"
+  super + s
+
 # Brightness (Mac Fn+F1/F2, also mapped as XF86 keys)
 "brightnessctl set 10%-"
   XF86MonBrightnessDown
@@ -684,6 +689,7 @@ file_permissions=(
   ["/usr/local/bin/oriondeskbar"]="0:0:755"
   ["/usr/local/bin/orionlauncher"]="0:0:755"
   ["/usr/local/bin/orionwallpaper"]="0:0:755"
+  ["/usr/local/bin/nebula"]="0:0:755"
   ["/usr/local/bin/orioninstall"]="0:0:755"
 )
 PROFILEDEF