feat(image-pipeline): add wasm simd fallback backend scaffold

lib/image-pipeline/backend/wasm/wasm-backend.ts

@@ -0,0 +1,37 @@
+import type {
+  BackendPipelineRequest,
+  BackendStepRequest,
+  ImagePipelineBackend,
+} from "@/lib/image-pipeline/backend/backend-types";
+import {
+  loadWasmKernelModule,
+  type WasmKernelModule,
+} from "@/lib/image-pipeline/backend/wasm/wasm-loader";
+
+type WasmBackendOptions = {
+  loadModule?: () => WasmKernelModule;
+};
+
+export function createWasmSimdBackend(options?: WasmBackendOptions): ImagePipelineBackend {
+  const loadModule = options?.loadModule ?? loadWasmKernelModule;
+  let kernelModule: WasmKernelModule | null = null;
+
+  function ensureModule(): WasmKernelModule {
+    if (kernelModule) {
+      return kernelModule;
+    }
+
+    kernelModule = loadModule();
+    return kernelModule;
+  }
+
+  return {
+    id: "wasm",
+    runPreviewStep(request: BackendStepRequest): void {
+      ensureModule().applyPreviewStep(request);
+    },
+    runFullPipeline(request: BackendPipelineRequest): void {
+      ensureModule().applyFullPipeline(request);
+    },
+  };
+}