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);
+    },
+  };
+}

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

@@ -0,0 +1,65 @@
+import type {
+  BackendPipelineRequest,
+  BackendStepRequest,
+} from "@/lib/image-pipeline/backend/backend-types";
+import { WASM_SIMD_PROBE_MODULE } from "@/lib/image-pipeline/backend/capabilities";
+import { applyPipelineStep, applyPipelineSteps } from "@/lib/image-pipeline/render-core";
+
+export type WasmKernelModule = {
+  applyPreviewStep: (request: BackendStepRequest) => void;
+  applyFullPipeline: (request: BackendPipelineRequest) => void;
+};
+
+let cachedModule: WasmKernelModule | null = null;
+
+function assertWasmSimdRuntimeSupport(): void {
+  if (typeof WebAssembly === "undefined") {
+    throw new Error("WebAssembly runtime is unavailable.");
+  }
+
+  if (typeof WebAssembly.validate !== "function") {
+    throw new Error("WebAssembly validation API is unavailable.");
+  }
+
+  if (!WebAssembly.validate(WASM_SIMD_PROBE_MODULE)) {
+    throw new Error("WebAssembly SIMD is unavailable.");
+  }
+
+  const module = new WebAssembly.Module(WASM_SIMD_PROBE_MODULE);
+  void new WebAssembly.Instance(module);
+}
+
+export function loadWasmKernelModule(): WasmKernelModule {
+  if (cachedModule) {
+    return cachedModule;
+  }
+
+  assertWasmSimdRuntimeSupport();
+
+  cachedModule = {
+    applyPreviewStep(request): void {
+      applyPipelineStep(
+        request.pixels,
+        request.step,
+        request.width,
+        request.height,
+        request.executionOptions,
+      );
+    },
+    applyFullPipeline(request): void {
+      applyPipelineSteps(
+        request.pixels,
+        request.steps,
+        request.width,
+        request.height,
+        request.executionOptions,
+      );
+    },
+  };
+
+  return cachedModule;
+}
+
+export function resetWasmKernelModuleCache(): void {
+  cachedModule = null;
+}