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/templates/app_shell.heml

@@ -4,7 +4,7 @@
   <meta charset="utf-8">
   <meta name="viewport" content="width=device-width, initial-scale=1">
   <title>hemx Kanban</title>
-  <script src="/hemx.js" defer></script>
+  <script +src="self.runtime_src" defer></script>
   <style>
     body { font-family: system-ui, sans-serif; margin: 2rem; }
     form { display: flex; gap: .5rem; flex-wrap: wrap; margin: 1rem 0; }