Register `rainlang-interface` on the Soldeer registry so the 0.2.0 publish succeeds

[thedavidmeister]

Blocker

Merging #116 (repo rename → rainlang.interface, soldeer-package → rainlang-interface, version → 0.2.0) triggered Package Release, but the Soldeer push failed:

Error: Failed to run soldeer: error during publishing: registry error during publishing:
project rainlang-interface not found. Please check the dependency name (project name) or
create a new project on https://soldeer.xyz

A brand-new Soldeer package name must be registered on soldeer.xyz before its first push — the old rain-interpreter-interface project exists, but rainlang-interface does not yet.

Action (manual, needs the Soldeer account owner)

1. Create the rainlang-interface project on https://soldeer.xyz under the org's account.
2. Re-run the failed Package Release run (or push any commit to main) so rainlang-interface@0.2.0 publishes.
3. Confirm rainlang-interface@0.2.0 appears on the registry.

Until this lands, the new-name package does not exist and consumers cannot migrate. The old rain-interpreter-interface@0.1.x revisions remain published, so nothing is broken in the meantime.

Follow-up to #105.

[thedavidmeister]

Resolved — the rainlang-interface Soldeer project was registered, and the re-triggered Package Release run (29312270814) published rainlang-interface@0.2.0 successfully. The 7 consumer-migration issues are now unblocked.