[nooby6]

Summary

• Correct the README milestone/status to reflect the existing PostgreSQL exclusion constraints.
• Document the btree_gist-backed half-open time-range invariant and the concurrent integration test.
• Replace the outdated claim that database-level consistency is future work with remaining coverage for concurrent updates and shift/leave interactions.

Validation

Documentation-only change. The underlying constraint and integration test are already on main from the previous consistency work.