feat(axum): expose hashed runtime asset path

Generate a SHA-256 content hash for the embedded hemx runtime, expose the hashed path from hemx-axum, serve immutable runtime headers, and update examples to load the hemx-owned path instead of fixed /hemx.js URLs.

req: axum_integration/005

hemx-js/build.rs

@@ -0,0 +1,21 @@
+use sha2::{Digest, Sha256};
+use std::env;
+use std::fs;
+use std::path::Path;
+
+fn main() {
+    let runtime_path = Path::new("runtime/hemx.js");
+    println!("cargo:rerun-if-changed={}", runtime_path.display());
+
+    let runtime = fs::read(runtime_path).expect("read hemx runtime");
+    let hash = format!("{:x}", Sha256::digest(&runtime));
+    let out_dir = env::var("OUT_DIR").expect("OUT_DIR");
+    fs::write(
+        Path::new(&out_dir).join("runtime_asset.rs"),
+        format!(
+            "pub const RUNTIME_JS_HASH: &str = \"{hash}\";\n\
+             pub const RUNTIME_JS_PATH: &str = \"/hemx.{hash}.js\";\n"
+        ),
+    )
+    .expect("write runtime asset constants");
+}