Mission-Neon: Initial creation

[pktoymail-arch]

Mission-Neon: Initial submission

Mission Neon is a dark, low-resource Cinnamon theme with a mission-control /
neon HUD style.

This PR adds one Cinnamon theme:

• UUID: Mission-Neon
• Author: pktoymail-arch
• License: GPLv3

Notes:

• The submitted spice contains only Cinnamon theme files and root metadata/
  images required by cinnamon-spices-themes.
• GTK, Metacity, wallpapers, icons, install scripts, and machine-specific
  configuration are intentionally excluded from this official Cinnamon Spices
  submission.
• The theme uses the lower-resource styling path: no box-shadow rules and
  no rgba(...) rules in the submitted Cinnamon CSS.

Checklist:

• Set author in Mission-Neon/info.json.
• Verify the theme under Cinnamon with top, bottom, left, and right panel
  positions.
• Confirm icon.png and screenshot.png are acceptable.