fix(player): stop logging six audio errors on every engine-backed load

Every track that plays through the Web Audio engine logged one
MEDIA_ELEMENT_ERROR "Empty src attribute" per stem, six lines, before the
user had done anything.

The elements really do have no source, and that is correct: when the engine
owns playback the multitrack is built with url: null for every stem, because
the engine streams the audio itself and the multitrack is only there for the
lanes. The guard that was supposed to skip those tested
stemsByName[name].url instead, which is the original descriptor and still
holds the real URL. So it passed, attached an error listener to an element
that was deliberately never given a src, and the element duly reported one.

Adds useEngine to the test, mirroring the condition that nulls the URLs in
the first place.

Playback was never affected. The cost was that anyone reading a console, or
attaching one to a bug report, saw six errors that had nothing to do with
their problem. It showed up while verifying #600 and is unrelated to it.

Author: Thales

static/js/player.js

@@ -1320,9 +1320,18 @@ export function wireUpAudio(jobId, stems, duration, thumbnail, mixUrl = null, ti
       `[player] canplay — ${stems.length} stems, ctx=${ctx?.state}, audios:`,
       mt.audios?.map((a, i) => `${orderedNames[i]}:${a?.constructor?.name}`),
     );
-    // Log load errors only for stems that actually have a source URL
+    // Log load errors only for stems that actually have a source URL.
+    //
+    // `useEngine` has to be part of that test, not just the stem descriptor.
+    // When the engine owns playback every multitrack stem is handed url: null
+    // above, so all of these elements have an empty src by design, while
+    // stemsByName still holds the real URL the engine is streaming from. Testing
+    // only the descriptor therefore passed, attached an error listener to an
+    // element that was never given a source, and logged six MEDIA_ELEMENT_ERROR
+    // "Empty src attribute" lines on every engine-backed track load. Harmless to
+    // playback, and noisy enough to bury a real error in a bug report.
     mt.audios?.forEach((a, i) => {
-      if (a instanceof HTMLMediaElement && stemsByName[orderedNames[i]]?.url) {
+      if (!useEngine && a instanceof HTMLMediaElement && stemsByName[orderedNames[i]]?.url) {
         a.addEventListener("error", () =>
           console.error(`[player] audio error stem[${i}] ${orderedNames[i]}:`, a.error?.message, a.error?.code),
         { once: true });