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/tests/e2e.rs

@@ -1,4 +1,4 @@
-use scraper::{Html, Selector};
+use hemx_axum::runtime_js_path;
 use hemx_techdemo::ui::control_center::{self as control, launch_work, reset_demo, simulate_push};
 use hemx_techdemo::ui::issue_card::{advance_work, delete_work, spotlight_work};
 use hemx_techdemo::ui::issue_lane::move_to_lane as move_to_lane_handle;
@@ -9,6 +9,7 @@ use hemx_test::{
     root_selector, sse_endpoint_marker, strong_text_selector, unknown_handle_form_body,
     EffectInspector,
 };
+use scraper::{Html, Selector};
 use std::io::{Read, Write};
 use std::net::TcpStream;
 use std::process::{Child, Command, Stdio};
@@ -73,7 +74,7 @@ fn product_is_e2e_working_over_http() {
     let favicon = get("/favicon.ico");
     assert_eq!(favicon.status, 204);
 
-    let runtime = get("/hemx.js");
+    let runtime = get(runtime_js_path());
     assert_eq!(runtime.status, 200);
     assert!(runtime.header("content-type").contains("javascript"));