feat: implement suspense-based alpha tracing for token meshes

Refactor `TokenMesh` to use a custom `useTrace` hook that suspends
during the alpha channel tracing process. This allows the `Bounds`
component in `Scene` to correctly fit the camera to the token's
geometry once it has been fully loaded and traced.

apps/web/src/components/viewers/Scene.tsx

@@ -1,6 +1,6 @@
 import { Suspense, type ReactNode } from 'react';
 import { Canvas } from '@react-three/fiber';
-import { ContactShadows, OrbitControls } from '@react-three/drei';
+import { Bounds, ContactShadows, OrbitControls } from '@react-three/drei';
 import { Bloom, EffectComposer, Vignette } from '@react-three/postprocessing';
 
 /**
@@ -8,6 +8,10 @@ import { Bloom, EffectComposer, Vignette } from '@react-three/postprocessing';
  * lighting, orbit controls, a soft contact shadow, and subtle post-processing
  * (bloom + vignette). Children are wrapped in a Suspense boundary so loading
  * assets (textures, models) can suspend without blanking the page.
+ *
+ * The camera is fitted to the bounds of the content on mount. `Bounds` sits
+ * inside the Suspense boundary, so it only mounts once the (suspending) content
+ * has loaded and its geometry is present.
  */
 export default function Scene({ children }: { children: ReactNode }) {
   return (
@@ -22,7 +26,9 @@ export default function Scene({ children }: { children: ReactNode }) {
         <directionalLight position={[-4, 2, -3]} intensity={0.4} color="#b3c7ff" />
         <pointLight position={[0, 3, 0]} intensity={0.3} />
 
-        <Suspense fallback={null}>{children}</Suspense>
+        <Suspense fallback={null}>
+          <Bounds>{children}</Bounds>
+        </Suspense>
 
         <ContactShadows
           position={[0, -0.5, 0]}

apps/web/src/components/viewers/TokenViewer.tsx

@@ -1,5 +1,5 @@
 import { useTexture } from '@react-three/drei';
-import { useEffect, useMemo, useState } from 'react';
+import { useMemo } from 'react';
 import * as THREE from 'three';
 import type { TTSObject } from '@tts/shared';
 import {
@@ -38,34 +38,11 @@ export default function TokenViewer({ object }: { object: TTSObject }) {
 function TokenMesh({ url, thickness }: { url?: string; thickness: number }) {
   const texture: THREE.Texture | null = url ? useTexture(assetUrl(url)) : null;
 
-  // Trace the image's alpha channel into a shape. Falls back to a circle while
-  // loading or when there's no image / the trace fails.
-  const [trace, setTrace] = useState<{
-    shape: { outline: number[][]; holes?: number[][][] };
-    width: number;
-    height: number;
-  } | null>(null);
-  useEffect(() => {
-    let cancelled = false;
-    setTrace(null);
-    if (!url) return;
-    traceImage(url, 'alpha', -TRACE_INSET)
-      .then((result) => {
-        if (!cancelled && result.shape) {
-          setTrace({
-            shape: result.shape,
-            width: result.width,
-            height: result.height,
-          });
-        }
-      })
-      .catch(() => {
-        if (!cancelled) setTrace(null);
-      });
-    return () => {
-      cancelled = true;
-    };
-  }, [url]);
+  // Trace the image's alpha channel into a shape. Suspends until the trace
+  // resolves so the surrounding Suspense boundary (and `Bounds`) only mounts
+  // once the token geometry is present. Falls back to a circle when there's no
+  // image or the trace fails.
+  const trace = useTrace(url);
 
   const { front, back, walls } = useMemo(() => {
     // The traced shape and its UV framing share the same transform, so the
@@ -99,6 +76,38 @@ function TokenMesh({ url, thickness }: { url?: string; thickness: number }) {
   );
 }
 
+interface TraceData {
+  shape: { outline: number[][]; holes?: number[][][] };
+  width: number;
+  height: number;
+}
+
+// Cache trace promises by URL so Suspense doesn't re-issue the request on every
+// render while the boundary is held open.
+const traceCache = new Map<string, Promise<TraceData | null>>();
+
+/**
+ * Suspend on the alpha trace for `url`, resolving to the traced shape (or null
+ * when there's no URL / the trace fails). Throwing a cached promise here lets
+ * the surrounding Suspense boundary hold rendering until the trace completes.
+ */
+function useTrace(url: string | undefined): TraceData | null {
+  if (!url) return null;
+  let promise = traceCache.get(url);
+  if (!promise) {
+    promise = traceImage(url, 'alpha', -TRACE_INSET).then((result) => {
+      if (!result.shape) return null;
+      return {
+        shape: result.shape,
+        width: result.width,
+        height: result.height,
+      } as TraceData;
+    });
+    traceCache.set(url, promise);
+  }
+  throw promise;
+}
+
 /** Convert raw extruded arrays into a three.js `BufferGeometry`. */
 function toGeometry(extruded: ExtrudedGeometry) {
   const { positions, normals, uvs, indices } = extruded;

apps/web/src/pages/ModPage.tsx

@@ -90,7 +90,9 @@ export default function ModPage() {
                       </div>
                     }
                   >
-                    <Viewer object={selected.object} />
+                    {/* Key by selection path so the Canvas remounts and the
+                        camera refits to the newly selected object. */}
+                    <Viewer key={selectedPath} object={selected.object} />
                   </Suspense>
                 </ErrorBoundary>
               )}