feat(mcp): expose entity creation schemas on demand

Expose creation fields, constraints and examples for the 11 supported types without expanding startup tool schemas. Share the creation contract and constructor with execution to keep accepted inputs consistent.

Validation: MCP, contract/factory and entity creation regression tests.

src/lib.rs

@@ -8,6 +8,7 @@ pub mod command;
 pub mod config;
 pub mod discussions;
 pub mod entities;
+pub(crate) mod entity_creation;
 #[cfg(not(target_arch = "wasm32"))]
 pub mod gpu_backend;
 pub mod i18n;
@@ -27,6 +28,7 @@ pub mod plugin;
 #[cfg(not(target_arch = "wasm32"))]
 pub mod rest;
 pub mod scene;
+pub(crate) mod schema_validation;
 pub mod snap;
 pub mod sys;
 pub mod sysinfo;