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/techdemo/templates/app_shell.heml

@@ -4,7 +4,7 @@
   <meta charset="utf-8">
   <meta name="viewport" content="width=device-width, initial-scale=1">
   <title>hemx Techdemo</title>
-  <script src="/hemx.js" defer></script>
+  <script +src="self.runtime_src" defer></script>
   <script src="/island.js" defer></script>
   <link rel="stylesheet" href="/app.css">
   <link rel="stylesheet" href="/control_center.css">