fix(studio): the repair runs off the page's own thread, and a stroke is answered twice so the hand is never held

The graph is seconds of solid arithmetic however large the crop is, and it ran
where the pointer lives: the brush lagged behind the hand for exactly as long as
the model took. The runtime's wasm provider now works in a worker of its own, so
the page keeps drawing while the repair is thought through.

The wait itself stays — it is the model's, not the page's — so a preview paints
the blurred fill first and lets the model's version of the same mask replace it.
Filing and export still wait for the model: the faster answer is for the eye
waiting on the brush and for nothing else.

Measured on the same photo and stroke, worst main-thread gap 3703 ms before and
487 ms after; previews land at 1.0 s and 5.3 s, and the model's render is the one
left on the stage.

docker/frontend/src/App.tsx

@@ -1032,6 +1032,30 @@ export function Workspace() {
         // render and the compare copy, so the two can never drift apart.
         const frame = cropEditing ? 'none' : frameId;
         const options = stageOptions;
+        // A repair in hand is drawn twice, and the first one is the cheap half:
+        // the blurred fill lands in a fraction of the model's fixed seconds and
+        // is a repair in its own right (it is what a machine without the runtime
+        // gets), so the stroke is answered at once and the model's version of
+        // the same mask replaces it when it arrives. Everything the app files or
+        // exports still waits for the model.
+        if (shownRecipe.adjustments.heal?.length) {
+          const quick = await renderPhoto({
+            source: previewBytes,
+            recipe: shownRecipe,
+            frameId: frame,
+            useGeotag,
+            gpsInfo,
+            assets: engine.assets,
+            options: { ...options, healModel: false },
+          });
+          if (token !== renderToken.current) return;
+          if (quick) {
+            setPreviewUrl((prev) => {
+              if (prev) URL.revokeObjectURL(prev);
+              return URL.createObjectURL(new Blob([quick.bytes as BlobPart], { type: 'image/jpeg' }));
+            });
+          }
+        }
         const out = await renderPhoto({
           source: previewBytes,
           recipe: shownRecipe,