Railbase core v0.5.21

Plugin manifest + universal install/configure/uninstall wizard (ADR-003).

• Plugins ship a manifest (UI mount/auth, settings, data policy, events, verb RBAC).
• Operators can override a plugin's mount, sign-in requirement, and settings, and map verb permissions to roles — via a new Configure dialog in the in-app marketplace.
• Settings groups (shareable across plugins, schema-compat-checked) resolved through $app.settings(group), with per-tenant overrides.
• Core-enforced verb→permission RBAC for data-resident plugin verbs.
• Uninstall now offers keep-data (dormant) vs delete-data (purge).
• Site shell honours per-plugin public vs app-user access.
• Removed the legacy railbase plugin setup CLI codegen wizard (superseded by install-time UI).
• Fixed an eventbus teardown race surfaced under -race.