Add maetning by soerenbnoergaard

[kmturley]

Summary

• Adds maetning, a saturation plugin with multiple selectable saturation algorithms.
• MIT. One zip bundles Mac/Win/Linux builds together, all x64-only (verified via file), kept as one multi-platform file entry.
• Added the missing VST2 format (so/dll) for Linux/Windows by hand — same "-vst" filename-substring-not-extension-boundary pattern as jpcima/stone-phaser (PR Add Stone Phaser by JP Cimalando #922), which the fetch script's detection doesn't match.

Test plan

• npx tsx src/validate.ts src/plugins/soerenbnoergaard/maetning/0.0.3/index.yaml passes

[github-actions]

VirusTotal Report:

• test/downloads/soerenbnoergaard/maetning/0.0.3/0-maetning-v0.0.3.zip

[github-actions]

Destination urls:
Registry: https://open-audio-stack.github.io/open-audio-stack-registry/plugins/soerenbnoergaard/maetning
Site: https://studiorack.github.io/studiorack-site/plugins/soerenbnoergaard/maetning

[github-actions]

Plugin author notification:

@soerenbnoergaard — your plugin has been submitted to the Open Audio Stack registry. Please review this PR and let us know if the metadata (name, description, tags, files) is correct. To see a full list of all currently added plugins, click here.