Logo (Builtin): adds LeonOS4

[VasilyZa]

Summary

Adds the ASCII logo for LeonOS 4 (requested in #2535).

Related issue (required for new logos for new distros)

Closes #2535

Changes

• Adds src/logo/ascii/l/leonos4.txt — the ASCII art from the request, using $1 / $2 placeholders mapped to the brand colors (FF_COLOR_FG_RGB "0;120;212" and FF_COLOR_FG_RGB "0;90;158").
• Registers it in src/logo/ascii/l.inc, appended at the end of the array like other recently added logos.
• Registered names:
  • LeonOS4 — the ID from /etc/os-release (ID=leonos4), so automatic detection works.
  • LeonOS — the LOGO= value from the same file, kept for manual selection (-l leonos). Since names are matched case-insensitively, listing leonos separately would be indistinguishable and is intentionally omitted.
• Adds a Logos entry to the unreleased section of CHANGELOG.md.

Note on #2536: that PR adds the same logo, but registers it as LeonOS, leonos and LeonOS 4. None of those equals ID=leonos4, so it would not be found by automatic detection, and the three names collapse into two distinct lookup keys because names are matched case-insensitively. This PR carries the identical artwork with the names corrected. Both are open — please merge whichever you prefer, and feel free to close the other.

Screenshots

$ fastfetch --list-logos | grep -i leonos
267) "LeonOS4" "LeonOS"

$ fastfetch --logo LeonOS4
     :-------:.
     :-----------.
     --------------.     .
    .---------------.  .::::
   .-----.:------:::..:::::::.
  :-----: -:::...::::::::::::::
 :-----: ...:::::::::::::::::::.
:-----. .:::::::::::...:::::::::
-----  .::::........::::::::::::
----  .:::  ..::::::::::::::::::
:--:  ::. .::::::::::::::::::::
 ---  :. .::::::::::::::::::::.
  :-. .  ::::::::::::::::::::
    :.   ::::::::::::::::::
         ..::::::::::::.

Automatic detection was verified against a simulated /etc/os-release (PRETTY_NAME="LeonOS 4", ID=leonos4): a plain fastfetch --structure logo picks this logo up without passing -l.

Verified locally: codespell (clean), cmake -DBUILD_TESTS=On + ctest (6/6 pass), fastfetch --format json (well-formed), fastfetch -c presets/all.jsonc --stat false (no errors), clang-format (entry matches the surrounding style of l.inc).

Checklist

• I have tested my changes locally.