R1/R2: Implement SDK-backed MCP extension on Restlet transports #1558

Follow-up to R0 draft PR #1556. MCP SDK 2.1.0 stateless HTTP/2, Streamable initialization/POST SSE, bounded GET notifications/replay and official STDIO round-trip are demonstrated. R0 does not authorize Ikanos migration: direct revision/session compatibility fails; the tools fixture matches 19/20 corpus responses, with one Jackson diagnostic difference and an explicit null-ID SDK restriction.

Acceptance: independent org.restlet.ext.mcp APIs; SDK-owned codecs/dispatch; explicit owned/borrowed connector lifecycle and finite byte/queue limits; supported module-path seams; strict revision/profile validation; R1 generic STDIO mechanics before R2 framing helpers; bounded Reactor bridges and cancellation; preserve no-session notifications/error semantics. Recheck complete profile, identity/capability checks and parser diagnostics before any migration. Production SSE/reconnect conformance remains R6. No common RPC API, duplicate dispatcher or SDK-owned HTTP engine. No Ikanos source-copy/header decision is needed for the R0 fixtures; agree attribution before any later copying.