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

Cargo.toml

@@ -20,6 +20,7 @@ solvecraft-doc = { path = "crates/doc" }
 solvecraft-io = { path = "crates/io" }
 solvecraft-engine = { path = "crates/engine" }
 solvecraft-ui-egui = { path = "crates/ui-egui" }
+solvecraft-mcp = { path = "crates/mcp" }
 
 # foundation
 thiserror = "2"