fix(refboard): reference-counted textures prevent shared source crash

When duplicated images share the same assetKey, Assets.unload()
was destroying the shared texture source — crashing all sprites
using it (null addressModeU/alphaMode in WebGL renderer).

TextureManager now tracks refCount per asset. Only calls
Assets.unload() when the last reference is released. ImageSprite
calls release() instead of unload() on texture teardown.

frontend/src/canvas/TextureManager.ts

@@ -2,15 +2,20 @@ import { Texture, Assets } from "pixi.js";
 
 interface TextureEntry {
   texture: Texture;
-  url: string; // needed for Assets.unload()
+  url: string;
+  refCount: number;
 }
 
 /**
- * TextureManager — GPU texture cache.
+ * TextureManager — GPU texture cache with reference counting.
  *
- * No internal LRU eviction — the viewport culling system (PixiCanvas)
- * manages loading/unloading based on proximity. This avoids destroying
- * textures that sprites still reference (which crashes PixiJS v8).
+ * Multiple ImageSprites can share the same assetKey (duplicated images).
+ * Assets.unload() destroys the shared texture source, so we must only
+ * call it when the LAST reference is released. Otherwise PixiJS v8
+ * crashes with null source errors (alphaMode, addressModeU).
+ *
+ * The viewport culling system (PixiCanvas) calls load/release to
+ * manage which textures consume GPU memory.
  */
 export class TextureManager {
   private cache = new Map<string, TextureEntry>();
@@ -24,12 +29,13 @@ export class TextureManager {
   }
 
   /**
-   * Load a texture for the given asset.
+   * Load a texture for the given asset. Increments ref count.
    * Returns cached texture if available, otherwise fetches and caches.
    */
   async load(assetKey: string): Promise<Texture> {
     const existing = this.cache.get(assetKey);
     if (existing) {
+      existing.refCount++;
       return existing.texture;
     }
 
@@ -43,15 +49,21 @@ export class TextureManager {
       return Texture.EMPTY;
     }
 
-    this.cache.set(assetKey, { texture, url });
+    this.cache.set(assetKey, { texture, url, refCount: 1 });
     return texture;
   }
 
-  /** Remove a specific texture from the cache via Assets.unload(). */
-  unload(assetKey: string): void {
+  /**
+   * Release a reference to a texture. Only actually unloads from GPU
+   * when the last reference is released.
+   */
+  release(assetKey: string): void {
     const entry = this.cache.get(assetKey);
     if (!entry) return;
 
+    entry.refCount--;
+    if (entry.refCount > 0) return; // other sprites still using it
+
     this.cache.delete(assetKey);
     try {
       Assets.unload(entry.url);
@@ -60,10 +72,22 @@ export class TextureManager {
     }
   }
 
+  /** Force unload (used by clear). */
+  unload(assetKey: string): void {
+    const entry = this.cache.get(assetKey);
+    if (!entry) return;
+    this.cache.delete(assetKey);
+    try {
+      Assets.unload(entry.url);
+    } catch {
+      // ignore
+    }
+  }
+
   /** Unload all cached textures. */
   clear(): void {
-    for (const [key] of this.cache) {
-      this.unload(key);
+    for (const [, entry] of this.cache) {
+      try { Assets.unload(entry.url); } catch { /* ignore */ }
     }
     this.cache.clear();
   }