Detect the language once for auto-detect file passes on the chunked path (D-053)

Transformers.js has no Whisper language detection and silently decoded
Danish as English whenever "Skip silence" was off or unavailable.

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

tests/live-real-model.test.ts

@@ -20,6 +20,7 @@ import {
 import { LONG_FORM_DEFAULTS, transcribeLongForm } from "../src/worker/longform";
 import { createWhisperDeps, loadSileroVad } from "../src/worker/longform/transformers";
 import { decodeClip } from "./clips";
+import { detectFileLanguage } from "../src/worker/language";
 import { wordErrorRate } from "./dsp";
 
 /**
@@ -500,3 +501,38 @@ describe("long-form file pipeline accuracy (real model, real recordings)", () =>
     );
   }
 });
+
+// ---------------------------------------------------------------------------
+// "Auto" language on the default chunked file path: Transformers.js has no
+// language detection and silently decodes as English, so the worker detects
+// once on the first 30 s (src/worker/language.ts) and pins it.
+// ---------------------------------------------------------------------------
+
+describe("auto language on the chunked file path (real model)", () => {
+  for (const clip of clips) {
+    test.skipIf(!available)(
+      `${clip.base}: detected as ${clip.language}, and the transcript stays close to the reference`,
+      async () => {
+        const reference = await Bun.file(join(AUDIO_DIR, clip.transcriptFile)).text();
+        const audio = await decodeClip(clip.audioFile);
+        const asr = await loadAsr();
+        const language = await detectFileLanguage(asr, audio);
+        expect(language).toBe(clip.language);
+        if (audio.length / RATE > 120) return; // detection is the point; long clips are covered above
+        const output = await asr(audio, {
+          chunk_length_s: 30,
+          stride_length_s: 5,
+          return_timestamps: true,
+          max_new_tokens: 440,
+          language,
+          task: "transcribe",
+        });
+        const text = ((Array.isArray(output) ? output[0] : output) as { text?: string }).text ?? "";
+        expect(wordErrorRate(normalize(reference), normalize(text))).toBeLessThanOrEqual(
+          MAX_WER_BY_CLIP[clip.base] ?? MAX_WER,
+        );
+      },
+      DOWNLOAD_BUDGET_MS + 60 * MS_PER_AUDIO_SECOND,
+    );
+  }
+});