feat(techdemo): add opaque island bridge

Add a canvas leaf-widget island to the techdemo that receives server snapshots through Effect::event/CustomEvent while keeping slhx core and runtime semantics unchanged.

req: interop/001

req: examples/001

req: examples/005

slhx-test/tests/examples_contract.rs

@@ -12,11 +12,9 @@ fn canonical_examples_do_not_author_browser_javascript() {
         }
         for (line_no, line) in text.lines().enumerate() {
             let trimmed = line.trim();
-            if trimmed.contains("<script")
-                && !trimmed.contains(r#"<script src="/slhx.js" defer></script>"#)
-            {
+            if trimmed.contains("<script") && !allowed_example_script(path, trimmed) {
                 failures.push(format!(
-                    "{}:{}: inline <script> is not allowed",
+                    "{}:{}: only the slhx runtime or explicit opaque-island scripts are allowed",
                     path.display(),
                     line_no + 1
                 ));
@@ -81,6 +79,13 @@ fn is_example_source(path: &PathBuf) -> bool {
     )
 }
 
+fn allowed_example_script(path: &Path, line: &str) -> bool {
+    // req: examples/005
+    line.contains(r#"<script src="/slhx.js" defer></script>"#)
+        || (path.ends_with("examples/techdemo/templates/app_shell.heml")
+            && line.contains(r#"<script src="/island.js" defer></script>"#))
+}
+
 fn contains_inline_event_handler(line: &str) -> bool {
     let bytes = line.as_bytes();
     let mut i = 0;