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/src/lib.rs

@@ -1,3 +1,5 @@
 pub const RUNTIME_ABI_VERSION: u32 = 1;
 pub const RUNTIME_JS: &str = include_str!("../runtime/hemx.js");
 pub const RUNTIME_D_TS: &str = include_str!("../runtime/hemx.d.ts");
+
+include!(concat!(env!("OUT_DIR"), "/runtime_asset.rs"));