Backwards compatibility & version negotiation #2892

Tracks: #2803

Make 2026-07-28 a real version the SDK can speak alongside every prior one: per-instance version knobs, the era classifier, and per-request _meta validation on the server. This is where the stateless server-side obligations land.

What's in it

• Per-instance "what versions do I serve/speak" knobs (server + client); add 2026-07-28 to the negotiable set.
• Era classifier + dispatch: server reads the _meta envelope (version/clientInfo/capabilities), validates it, emits the unsupported-version / missing-capability errors with typed data, and skips the legacy init gate for modern traffic.
• Era-gate everything removed from core for modern peers: ping, logging/setLevel, roots/list_changed, the tasks surface, the URL-elicitation-required error.
• resultType stamped on every modern result (the strip-for-legacy half is already in Protocol types for 2026-07-28: superset monolith, committed per-version packages, and wire-method maps #2849).

Conformance

• (no scenario passes on this alone — it's the foundation the others stand on)
• Together with New HTTP serving path (modern + dual-era), Capabilities API + server/discover handler, subscriptions/listen, Context object (layered, per-request): server-stateless
• Together with New Client (all-versions, low-level + high-level) #2894: request-metadata

Dependencies

• Depends on: none

References

• Versioning
• SEP-2575
• SEP-2567
• sep-2575.yaml