ci: add IDL freshness check and consolidate artifacts

Move IDL files to artifacts/ and add a convention-based CI check that
discovers all programs via */methods/guest/src/bin/*.rs and fails if
any program is missing its IDL or has one that is out of date.

tools/idl-gen/Cargo.toml

@@ -0,0 +1,14 @@
+[package]
+name = "idl-gen"
+version = "0.1.0"
+edition = "2021"
+
+[[bin]]
+name = "idl-gen"
+path = "src/main.rs"
+
+[dependencies]
+spel-framework-core = { git = "https://github.com/logos-co/spel.git", rev = "57201f64b4542bb3f592bc9a0aa654c47aa908a6", features = [
+  "idl-gen",
+] }
+serde_json = "1.0"

tools/idl-gen/src/main.rs

@@ -0,0 +1,19 @@
+use std::{path::PathBuf, process};
+
+fn main() {
+    let path: PathBuf = match std::env::args().nth(1) {
+        Some(p) => PathBuf::from(p),
+        None => {
+            eprintln!("Usage: idl-gen <source-file>");
+            process::exit(1);
+        }
+    };
+
+    match spel_framework_core::idl_gen::generate_idl_from_file(&path) {
+        Ok(idl) => println!("{}", serde_json::to_string_pretty(&idl).unwrap()),
+        Err(e) => {
+            eprintln!("Error: {e}");
+            process::exit(1);
+        }
+    }
+}