Migrate an existing app to Rspack

Rspack compiles app code; Meteor builds Atmosphere packages and assembles the bundle. Keep packages, but audit their architecture and browser dependencies.

Meteor 3.4+ required. Prefer enabling "meteor": { "modern": true } and fixing Babel fallbacks first. For setup/helpers, use meteor-modern-build-stack.

Match @meteorjs/rspack to the Meteor release, not to @rspack/core or @rspack/cli: