Logging: per-request log level on modern servers (era fork) #1629

Stack: SDK V2 + New Spec — 6 of 10 (tab era-fork; needs era model from card 3).

Fork the Logging UX by era. On modern servers logging/setLevel and the session-scoped level are gone: the client opts into logs per request via _meta["io.modelcontextprotocol/logLevel"], and servers MUST NOT emit notifications/message for requests that omitted it. The SDK does not auto-attach logLevel — server logs are silently absent until we stamp it.

Scope

• Legacy era: keep the current LogControls level Select + "Set" button → logging/setLevel, unchanged.
• Modern era: replace the session-level selector with a per-request (or global "stamp every request") log-level control; make clear in the UI that logs arrive on the originating request's stream. Gate the legacy selector out on modern.
• ping UI, if any, is legacy-only.

Key files

clients/web/src/components/screens/LoggingScreen/*, groups/LogControls/*, core/mcp/inspectorClient.ts (per-request _meta stamping), core/mcp/state/messageLogState.ts.

Acceptance

• Legacy servers: level selector works as today.
• Modern servers: logs appear only when the client stamps logLevel; UI reflects per-request semantics; setLevel selector hidden.
• npm run ci green.

Ref: specification/v2_new_spec_impact.md §7.6, §8.3(log level), §9.3(Logs), §9.5(5).