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

examples/kanban/src/lib.rs

@@ -5,9 +5,9 @@ pub mod ui {}
 mod tests {
     use super::ui::{board, board_card};
     use hemplate::Hemplate;
-    use scraper::{Html, Selector};
     use hemx::IntoEffect;
     use hemx_test::inspect;
+    use scraper::{Html, Selector};
 
     #[derive(Hemplate)]
     #[hemplate = "partials"]