fix(release): publish Nexus independently

[smackgg]

Summary

• prepare Nexus Service 0.3.12
• replace the repository-local release selector with Marketplace Kit 0.1.1
• preserve unselected production Registry/Showcase packages instead of forcing source-ahead siblings to publish
• verify v1/v2 Registry, Showcase, product-lock, inherited assets, and exact Release directory closure
• fetch all five production endpoints atomically with rerun-safe cache keys

Expected publication

The clean committed selector emits exactly:

[{"kind":"plugin","id":"nexus-service","version":"0.3.12","previousVersion":"0.3.11","releaseTag":"plugin-nexus-service-v0.3.12"}]

A production v1 bootstrap simulation advances sequence 49 → 50 while preserving 27 packages, 13 showcases, and the production JianYing version; only Nexus plus Registry metadata are newly released.

Validation

• bun run check
• 150 tooling tests / 982 expectations
• 30 focused release closure tests / 88 expectations
• production five-endpoint fetch and selective build
• product-lock verification with inherited FFmpeg Plugin, owned Skill, and companion bytes
• git diff --check

External prerequisite

@convax/marketplace@0.1.1 and @convax/marketplace-kit@0.1.1 must be staged on npm before merge. The protected npm workflow is currently prevented from starting by the repository account billing/spending-limit state; its jobs fail with zero executed steps.