[datacas]

Updates the M4 row of the milestone table in docs/ROADMAP_V2.md, as AGENTS.md requires when a milestone's content changes.

• States that the first delivery of M4 is descriptive review observability.
• Adds Evaluate reviewer findings with a reproducible offline A/B benchmark #391 (P2): an offline reviewer benchmark with an informative per-case A/B and independent labels, as an independent experimental delivery that neither blocks the main candidate nor depends on Run approved controlled evaluations across distinct work items #126, Produce comparable incremental code-quality evidence for existing reviews #331 or Import attributable production deployment and incident events #335.

Documentation only; the behaviour is already documented in docs/. Refs #391. This PR closes no issue, so it carries no closing keyword.