Add ImageSprite with LOD tier switching and lazy loading

Introduces ImageSprite (extends Sprite) that manages per-image LOD
texture swapping via TextureManager. Shows a placeholder rect until
the first texture loads, then upgrades/downgrades tiers based on zoom.

frontend/src/canvas/sprites/ImageSprite.ts

@@ -0,0 +1,93 @@
+import { Sprite, Texture, Graphics } from "pixi.js";
+import { TextureManager, LODTier } from "../TextureManager";
+
+/**
+ * A Sprite subclass that manages LOD tier switching and lazy loading.
+ * Shows a placeholder shimmer rect until the first texture tier loads,
+ * then swaps textures as zoom level changes.
+ */
+export class ImageSprite extends Sprite {
+  readonly assetKey: string;
+
+  private textures: TextureManager;
+  private currentTier: LODTier | null = null;
+  private loading = false;
+  private placeholder: Graphics | null = null;
+  private _naturalWidth: number;
+  private _naturalHeight: number;
+
+  constructor(
+    assetKey: string,
+    w: number,
+    h: number,
+    textures: TextureManager,
+  ) {
+    super(Texture.EMPTY);
+
+    this.assetKey = assetKey;
+    this.textures = textures;
+    this._naturalWidth = w;
+    this._naturalHeight = h;
+
+    this.width = w;
+    this.height = h;
+
+    // Create placeholder: dark rect shown until first texture loads
+    const placeholder = new Graphics();
+    placeholder.rect(0, 0, w, h).fill(0x2a2a2a);
+    this.placeholder = placeholder;
+    this.addChild(placeholder);
+
+    // Immediately start loading the thumbnail tier
+    this.loadTier("thumb");
+  }
+
+  /**
+   * Load a specific LOD tier texture for this sprite.
+   * Skips if already at the requested tier or currently loading.
+   */
+  async loadTier(tier: LODTier): Promise<void> {
+    if (this.currentTier === tier || this.loading) return;
+
+    this.loading = true;
+    try {
+      const tex = await this.textures.load(this.assetKey, tier);
+      this.texture = tex;
+      this.width = this._naturalWidth;
+      this.height = this._naturalHeight;
+      this.currentTier = tier;
+
+      // Remove placeholder after first successful load
+      if (this.placeholder) {
+        this.removeChild(this.placeholder);
+        this.placeholder.destroy();
+        this.placeholder = null;
+      }
+    } catch (err) {
+      console.warn(
+        `[ImageSprite] Failed to load tier "${tier}" for "${this.assetKey}":`,
+        err,
+      );
+    } finally {
+      this.loading = false;
+    }
+  }
+
+  /**
+   * Evaluate the current zoom level and switch LOD tier if needed.
+   */
+  updateLOD(zoom: number): void {
+    const needed = this.textures.tierForZoom(zoom);
+    if (needed !== this.currentTier) {
+      this.loadTier(needed);
+    }
+  }
+
+  /**
+   * Release the current texture (for off-screen sprites to free GPU memory).
+   */
+  unloadTexture(): void {
+    this.texture = Texture.EMPTY;
+    this.currentTier = null;
+  }
+}