MCP: server and tools

crates/mcp (L5, engine only): JSON-RPC 2.0 over stdio with protocol version negotiation, a
headless Session backend and a bridge to the app's control port, 14 tools (list_commands,
execute, batch with rollback, inspect_design, measure, body_topology, set_parameter,
screenshot as an image block, export, undo/redo, new_design/open/save) and tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

xtask/src/layers.rs

@@ -43,6 +43,7 @@ pub const TABLE: &[(&str, Class)] = &[
     ("io", Class::Layer(3)),
     ("engine", Class::Layer(4)),
     ("ui-egui", Class::Layer(5)),
+    ("mcp", Class::Layer(5)),
     ("testkit", Class::Testkit),
     // apps and tooling
     ("solvecraft", Class::Exempt),