Serve both MCP eras from the v2 stack on neutral hosts

[RhysSullivan]

First consolidation step: the neutral hosts (selfhost + local) now serve BOTH protocol eras from the SDK v2 stack — the v1 assembly is no longer used outside the Cloudflare hosts.

• in-memory-session-store.ts builds sessions on the v2 WebStandardStreamableHTTPServerTransport + the v2 assembly (sessionful mode: apps capability and elicitation support follow the live initialize-negotiated state, preserving the cold-restore semantics).
• Native elicitation for legacy clients rides the SDK's built-in legacy shim (inputRequired returns become real elicitation/create requests); native sessions use SSE responses as the shim requires, model/browser sessions keep JSON.
• apps/local stdio moves to serveStdio — one process now serves legacy AND modern stdio clients (deferral removed).
• selfhost drops its v1 SDK dependency entirely; local's is dev-only (compat tests).

Interop evidence (new tests): a real v1-SDK client against the consolidated store — initialize, tools/list with negotiated Apps tools, sessionCount() === 1 on reuse, and the full native-elicitation round-trip (elicitation/create emitted by the shim, response accepted, execution completes). stdio covers both a legacy v1 client and a modern-pinned v2 client.

Tests: 204 hosts/mcp + 72 local + 102 selfhost; no existing behavioral assertion changed.

Next: the cloud DO rewrite off McpAgent (deletes the 911-line agents patch), then removal of the v1 assembly and the V2 naming.

Stacked on #1604.

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commit: 074a9be

[RhysSullivan]

Folded into #1611 (single consolidated PR per review preference).