feat: add public Checkmate Launcher discovery metadata

[Siel]

Summary

• Add public LAPKB Launcher discovery metadata for the current Checkmate desktop app on bootstrap.
• Retain logical catalog ID checkerboard; use the existing Checkmate, org.lapkb.checkmate, checkmate-desktop identity and current icon.
• Document public opening and the distinction between discovery and Install/Update support.

This does not change scientific code, branding implementation, app versions, licensing, or release workflows. The obsolete Checkerboard Bliss bundle is not an alias for the new bundle.

Verification

• App manifest validated against the app-source-v1 JSON Schema using AJV 2020.
• Product/bundle identity, Cargo executable alias, referenced paths and PNG icon checks passed.
• Companion Launcher catalog schema and byte-for-byte regeneration checks passed.
• Companion Launcher frontend build and 58 synthetic browser tests passed.
• Bounded metadata/catalog review found no blockers; git diff --check passed.

No scientific rerun or installed-app acceptance is claimed for this metadata-only change. The companion Launcher catalog PR will carry native discovery regression results.

Rollout

Target: bootstrap, as requested by the project owner. A Launcher build with the regenerated catalog is required to discover the renamed bundle. No installer source, signed release archive or update feed is added here; install Checkmate independently until those are verified separately.