chore: add closed-beta Gantt chart

Summary

A new "Delivery Timeline" section has been added to the closed beta roadmap document. It sets the official internal planning forecast: package development is expected to wrap up around July 30, followed by integration and readiness work through mid-August, with a credible closed beta targeting the week of August 17. The section explicitly rules out June and flags July 16 as too optimistic. It includes a milestone table and a visual Gantt chart covering all workstreams — foundations, authentication, package formats (Container/OCI, npm, Maven), frontend, CLI, lifecycle, and integration — with notes on which items sit on the critical path and where the biggest scheduling risks lie.