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, 11 insertions, 7 deletions

diff --git a/webAO/utils/fileExists.ts b/webAO/utils/fileExists.ts
index 1dceb72..748bc1f 100644
--- a/webAO/utils/fileExists.ts
+++ b/webAO/utils/fileExists.ts
@@ -1,14 +1,15 @@
-export default async function fileExists(url: string): Promise<boolean> {
-  return new Promise((resolve) => {
+const cache = new Map<string, Promise<boolean>>();
+
+export default function fileExists(url: string): Promise<boolean> {
+  const cached = cache.get(url);
+  if (cached !== undefined) return cached;
+
+  const promise = new Promise<boolean>((resolve) => {
     const xhr = new XMLHttpRequest();
     xhr.open("HEAD", url);
     xhr.onload = function checkLoad() {
       if (xhr.readyState === 4) {
-        if (xhr.status === 200) {
-          resolve(true);
-        } else {
-          resolve(false);
-        }
+        resolve(xhr.status === 200);
       }
     };
     xhr.onerror = function checkError() {
@@ -16,4 +17,7 @@ export default async function fileExists(url: string): Promise<boolean> {
     };
     xhr.send(null);
   });
+
+  cache.set(url, promise);
+  return promise;
 }