Macro mcp_tool 

macro_rules! mcp_tool {
    ($fn_name:ident, $order:expr, $desc:literal, $param:ty,
     tips = [$($tip:literal),* $(,)?],
     |$srv:ident, $p:ident| async $body:block
    ) => { ... };
}

Define an MCP tool: route builder fn plus inventory metadata.