refactor(highway): flip highway.js to an ES module (R3c)

static/highway.js

@@ -1498,7 +1498,7 @@
         } catch (e) {
             hwState._rendererDrawFailures += 1;
             console.error('renderer draw:', e);
             // Self-heal: a plugin whose draw() throws every frame
             // would otherwise spam the console and leave the canvas
             // blank indefinitely. After a short streak of failures,
             // revert to the built-in renderer so the user at least
@@ -4240,6 +4240,21 @@
 }
 const highway = createHighway();
 window.highway = highway; // expose for plugins
+
+// THE FACTORY IS PART OF THE PUBLIC CONTRACT, and a classic script gave it to us for free.
+//
+// A top-level `function createHighway()` in a CLASSIC script implicitly becomes
+// window.createHighway. In a MODULE it does not — module declarations are module-scoped, and
+// the name would silently vanish from the global object the moment this file grew a
+// type="module" attribute. The constitution names window.createHighway as public extension
+// contract (alongside window.playSong / showScreen / feedBack), and plugins that render their
+// own highway panel construct a second instance with it. Nothing in-tree calls it, which is
+// exactly why this would have shipped: the only consumers are third-party, and I cannot grep
+// those.
+//
+// So it is assigned explicitly now. Same object, same behaviour, no longer an accident of how
+// the file happens to be loaded.
+window.createHighway = createHighway;
 highway.setOnLyricsChange(function(visible) {
     const btn = document.getElementById('btn-lyrics');
     if (btn) {

static/v3/index.html

@@ -1241,7 +1241,7 @@ <h3>Gameplay Settings</h3>
     </main>
     <!-- /#v3-main -->
 
-    <script defer src="/static/highway.js"></script>
+    <script type="module" src="/static/highway.js"></script>
     <script defer src="/static/vendor/lottie.min.js"></script>
     <script defer src="/static/lottie-api.js"></script>
     <script type="module" src="/static/app.js"></script>