Better tests and more user feedback while doing transcription; WIP.

tests/protocol.test.ts

@@ -1,6 +1,7 @@
 import { describe, test, expect } from "bun:test";
 import {
   applyLoadProgress,
+  deviceAttempts,
   initialLoadProgress,
   isUiToWorkerMessage,
   isWorkerToUiMessage,
@@ -103,6 +104,57 @@ describe("isWorkerToUiMessage", () => {
   });
 });
 
+describe("deviceAttempts (D-004, D-017)", () => {
+  test("auto resolves WebNN → WebGPU → WASM when everything is available", () => {
+    expect(deviceAttempts("auto", { webnn: true, webgpu: true })).toEqual([
+      "webnn",
+      "webgpu",
+      "wasm",
+    ]);
+  });
+
+  test("auto skips backends the probe ruled out, in order", () => {
+    expect(deviceAttempts("auto", { webnn: false, webgpu: true })).toEqual([
+      "webgpu",
+      "wasm",
+    ]);
+    expect(deviceAttempts("auto", { webnn: true, webgpu: false })).toEqual([
+      "webnn",
+      "wasm",
+    ]);
+    expect(deviceAttempts("auto", { webnn: false, webgpu: false })).toEqual(["wasm"]);
+  });
+
+  test("explicit webgpu falls back to WASM only", () => {
+    expect(deviceAttempts("webgpu", { webnn: true, webgpu: true })).toEqual([
+      "webgpu",
+      "wasm",
+    ]);
+    expect(deviceAttempts("webgpu", { webnn: true, webgpu: false })).toEqual(["wasm"]);
+  });
+
+  test("explicit webnn falls back to WASM only (no WebGPU detour)", () => {
+    expect(deviceAttempts("webnn", { webnn: true, webgpu: true })).toEqual([
+      "webnn",
+      "wasm",
+    ]);
+    expect(deviceAttempts("webnn", { webnn: false, webgpu: true })).toEqual(["wasm"]);
+  });
+
+  test("wasm is honored as-is and every chain ends in WASM", () => {
+    expect(deviceAttempts("wasm", { webnn: true, webgpu: true })).toEqual(["wasm"]);
+    for (const preference of ["auto", "webnn", "webgpu", "wasm"] as const) {
+      for (const webnn of [true, false]) {
+        for (const webgpu of [true, false]) {
+          const attempts = deviceAttempts(preference, { webnn, webgpu });
+          expect(attempts.length).toBeGreaterThan(0);
+          expect(attempts.at(-1)).toBe("wasm");
+        }
+      }
+    }
+  });
+});
+
 describe("applyLoadProgress", () => {
   test("tracks per-file byte progress without mutating previous state", () => {
     const p1: LoadProgressMessage = { type: "load-progress", status: "initiate", file: "a.onnx" };