Run browser file transcription through the long-form pipeline when asked and the VAD model is available

Adds an Advanced "Skip silence in uploaded files" option (off by default
pending measurement), the vad/longForm protocol fields, the old-path
fallback when the VAD model is missing, and a plain message for files
with no speech.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/protocol.test.ts

@@ -4,6 +4,7 @@ import {
   deviceAttempts,
   dtypeRequiresF16,
   initialLoadProgress,
+  choosePipeline,
   isUiToWorkerMessage,
   isWorkerToUiMessage,
   TranscribeProgressTracker,
@@ -35,7 +36,9 @@ describe("isUiToWorkerMessage", () => {
       { type: "transcribe", audio: new Float32Array(8), requestId: "live-3" },
       { type: "transcribe", audio: new Float32Array(8), requestId: "engine-1-1", stream: true },
       { type: "transcribe", audio: new Float32Array(8), stream: false },
+      { type: "transcribe", audio: new Float32Array(8), stream: true, longForm: true },
     ];
+    loads.push({ type: "load", modelId: "onnx-community/whisper-base", device: "auto", vad: true });
     for (const message of [...loads, ...transcribes]) {
       expect(isUiToWorkerMessage(message)).toBe(true);
     }
@@ -65,10 +68,26 @@ describe("isUiToWorkerMessage", () => {
     expect(
       isUiToWorkerMessage({ type: "transcribe", audio: new Float32Array(4), stream: "yes" }),
     ).toBe(false);
+    expect(
+      isUiToWorkerMessage({ type: "transcribe", audio: new Float32Array(4), longForm: 1 }),
+    ).toBe(false);
+    expect(isUiToWorkerMessage({ type: "load", modelId: "x", device: "auto", vad: "yes" })).toBe(
+      false,
+    );
     expect(isUiToWorkerMessage({ type: "result", text: "", chunks: [] })).toBe(false);
   });
 });
 
+describe("choosePipeline (D-WS3a/D-WS3b)", () => {
+  test("long-form only when asked for and the VAD model loaded", () => {
+    expect(choosePipeline({ longForm: true, vadReady: true })).toBe("longform");
+    // Offline / never downloaded: the old path, never an error (Review Focus 3).
+    expect(choosePipeline({ longForm: true, vadReady: false })).toBe("chunked");
+    expect(choosePipeline({ longForm: false, vadReady: true })).toBe("chunked");
+    expect(choosePipeline({ longForm: undefined, vadReady: true })).toBe("chunked");
+  });
+});
+
 describe("isWorkerToUiMessage", () => {
   test("accepts every worker → UI message shape", () => {
     const messages: WorkerToUiMessage[] = [