feat(http): implement to_mcp 4-tool gateway projection (rmcp ServerHandler, StreamableHttpService at /mcp)

to_mcp is the MCP-direction gateway projection (ADR-041): exposes 4 fixed
gateway tools (search, schema, call, batch) over rmcp StreamableHttpService
nested into the axum Router at /mcp, not one MCP tool per registry operation.
The LLM discovers operations on demand via search+schema.

- ToMcpGateway implements rmcp ServerHandler (call_tool, list_tools, get_info)
- tools/list returns the 4 fixed gateway tools, never the registry's ops
- search dispatches services/list via GatewayDispatch::invoke, excludes
  Subscription ops (ADR-041 §2), returns names + descriptions
- schema dispatches services/schema, returns the full OperationSpec
- call dispatches via GatewayDispatch::invoke (shared spine), maps
  ResponseEnvelope -> CallToolResult::structured (Ok) /
  CallToolResult::structured_error (Err(CallError))
- batch loops over invoke, returns an array of results
- Bearer auth via shared bearer_auth_middleware applied around nest_service
  (rmcp simple_auth_streamhttp pattern); Identity read from
  RequestContext.extensions -> http::request::Parts.extensions
  (research §6 #2 identity-survives-framing assumption, confirmed via test)
- to_mcp is a pure projection (consumes registry, produces no entries)
- Feature-gated behind mcp; stdio NOT built (ADR-037)
- /mcp route wired in adapter.rs replacing the placeholder 501

cargo test -p alknet-http --features mcp: 172 passed
cargo clippy -p alknet-http --features mcp --all-targets: clean
cargo check -p alknet-http (no mcp): clean

crates/alknet-http/src/adapters/mod.rs

@@ -13,7 +13,13 @@ pub mod from_openapi;
 #[cfg(feature = "mcp")]
 pub mod from_mcp;
 
+#[cfg(feature = "mcp")]
+pub mod to_mcp;
+
 pub use from_openapi::{FromOpenAPI, HttpAuthScheme, HttpServiceConfig, OpenAPISpec};
 
 #[cfg(feature = "mcp")]
 pub use from_mcp::FromMCP;
+
+#[cfg(feature = "mcp")]
+pub use to_mcp::{ToMcpGateway, ToMcpService, to_mcp_service};