Add asset preloading system for IC message rendering

Fix rendering race conditions where character sprites, pre-animations,
and paired character assets were displayed before being downloaded.
All assets referenced in an MS packet are now resolved and preloaded
into the browser cache before the animation timeline starts.

- Add unified assetCache module with session-wide promise caching
- Add preloadMessageAssets orchestrator for parallel asset resolution
- Cache fileExists HEAD requests so missing files aren't re-probed
- Preload all SFX (emote, shout, realization, stab) alongside sprites
- Use synchronous setEmoteFromUrl at all render transition points
- Graceful fallback to legacy setEmote if preloading times out

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

1 files changed, 21 insertions, 0 deletions

diff --git a/webAO/client/setEmoteFromUrl.ts b/webAO/client/setEmoteFromUrl.ts
new file mode 100644
index 0000000..8075bbd
--- /dev/null
+++ b/webAO/client/setEmoteFromUrl.ts
@@ -0,0 +1,21 @@
+import transparentPng from "../constants/transparentPng";
+
+/**
+ * Sets a pre-resolved emote URL on the correct DOM <img> element.
+ * This is synchronous because the image should already be in the browser cache
+ * from preloading.
+ */
+const setEmoteFromUrl = (url: string, pair: boolean, side: string): void => {
+  const pairID = pair ? "pair" : "char";
+  const acceptedPositions = ["def", "pro", "wit"];
+  const position = acceptedPositions.includes(side) ? `${side}_` : "";
+  const emoteSelector = document.getElementById(
+    `client_${position}${pairID}_img`,
+  ) as HTMLImageElement;
+
+  if (emoteSelector) {
+    emoteSelector.src = url || transparentPng;
+  }
+};
+
+export default setEmoteFromUrl;