Add BSEQuencer by sjaehn

[kmturley]

Summary

• Adds BSEQuencer, a multi-channel MIDI step sequencer (LV2) with a variable-size pattern matrix (up to 32x16), per-pad randomization, and a built-in scale/drumkit editor.
• GPL-3.0. Dropped the redundant .zip release asset — verified via tar/unzip it's byte-identical content to the .tar.xz, just a second archive format for the same Linux x64 build.
• type: generator (a MIDI pattern sequencer, not an audio-generating instrument).

Test plan

• npx tsx src/validate.ts src/plugins/sjaehn/bsequencer/1.8.10/index.yaml passes

[github-actions]

VirusTotal Report:

• test/downloads/sjaehn/bsequencer/1.8.10/0-BSEQuencer-1.8.10-linux-amd64.lv2.tar.xz

[github-actions]

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

[github-actions]

Plugin author notification:

@sjaehn — 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.