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, 20 insertions, 0 deletions

diff --git a/webAO/viewport/interfaces/PreloadedAssets.ts b/webAO/viewport/interfaces/PreloadedAssets.ts
new file mode 100644
index 0000000..54d20b8
--- /dev/null
+++ b/webAO/viewport/interfaces/PreloadedAssets.ts
@@ -0,0 +1,20 @@
+export interface PreloadedAssets {
+  /** Resolved URL for idle (a) sprite */
+  idleUrl: string;
+  /** Resolved URL for talking (b) sprite */
+  talkingUrl: string;
+  /** Resolved URL for pre-animation sprite (no prefix) */
+  preanimUrl: string;
+  /** Duration of preanim in ms (0 if no preanim) */
+  preanimDuration: number;
+  /** Resolved URL for paired character idle (a) sprite */
+  pairIdleUrl: string;
+  /** Resolved per-character shout SFX URL, or null to use default */
+  shoutSfxUrl: string | null;
+  /** Resolved emote SFX URL, or null if no sound */
+  emoteSfxUrl: string | null;
+  /** Resolved realization (flash) SFX URL */
+  realizationSfxUrl: string | null;
+  /** Resolved stab (screenshake) SFX URL */
+  stabSfxUrl: string | null;
+}