experimental-ext-variants

Summary Builds on the types and architecture from the previous PR to deliver a working variant-aware MCP server that routes requests to inner mcp.Server instances based on per-request _meta variant selection, enabling a single MCP endpoint to serve multiple tool/resource/prompt variants simultaneously. Key additions Backend abstraction (inMemoryBackend) — creates in-memory transport pairs between the variant server and inner mcp.Server instances, with notification forwarding for progress and logging events. Request dispatcher (dispatch.go) — handles routing for tools/list, tools/call, resources/list, resources/read, resources/subscribe, prompts/list, prompts/get, and completion/complete methods. Implements variant-scoped cursor wrapping to correctly paginate across multiple inner servers. Session middleware (session.go) — manages per-session inner connections in stateful mode and shared connections in stateless HTTP mode, ensuring proper lifecycle management. Initialize enrichment — injects availableVariants into the server's capabilities response so clients can discover which variants are available. Capability discovery — merges inner server capabilities via union to present a unified capability set to clients. NewStreamableHTTPHandler — provides an HTTP handler for multi-client serving over Streamable HTTP transport. Integration tests (server_test.go, 458 lines) — covers end-to-end variant selection, backward compatibility (no variant specified), HTTP transport, and stateless modes. Examples — stdio-based (examples/server/variants/main.go) and HTTP-based (examples/server/variants-http/main.go) example servers demonstrating the API. README — rewritten with API documentation and architecture diagrams. TODO (subsequent PRs): Unit Testing More integration tests