feat(sdk): rename attribute to #[chain_module], keep #[fai_module] alias

The public macro name was the last product-scoped 'fai' identifier
in the SDK surface. #[chain_module] is now the primary attribute
(README + examples updated); #[fai_module] stays as a delegating
alias so modules written before the rename keep compiling. 0.2.1.

Signed-off-by: flemming-it <sf@flemming.it>

examples/echo/src/lib.rs

@@ -1,13 +1,13 @@
 //! Minimum-viable F∆I module — echoes the first text input.
 //!
 //! Demonstrates the entire authoring surface: a single function
-//! decorated with `#[fai_module]`. No `wit_bindgen`, no
+//! decorated with `#[chain_module]`. No `wit_bindgen`, no
 //! `unsafe_op_in_unsafe_fn` allow, no `export!` call, no manual
 //! Payload-variant matching.
 
 use chain_module_sdk::prelude::*;
 
-#[fai_module]
+#[chain_module]
 fn invoke(_ctx: Context, inputs: Inputs) -> Result<Outputs, ModuleError> {
     let text = inputs.require_text("input")?;
     Ok(Outputs::new().with_text("output", format!("echo: {text}")))