fix(producer): classify the audio failure that takes the whole mix down

`runAudioStage` catches the rejection `processCompositionAudio` throws when an
FX failure cannot be degraded past — and returned `audioFailures: undefined`
with it. `applyDistributedAudioWarningPolicy` reads owner, retryability, reason
and stage off that list, so the FATAL failure was reported with an empty
reasons array, an empty stages array and no owner: strictly less classification
than a single dropped track gets, which is the opposite of what the stage's own
comment says it exists to do.

A failure is now synthesised from the error. "internal" is the honest bucket —
the stage names enumerate ffmpeg steps and this is the FX render, which is none
of them — and system/non-retryable is right for a browser or chain that will
not build. The detail is bounded at 2000 characters like the mixer's own.

Falsified by restoring `audioFailures: undefined`: both assertions fail.

Producer's suite has 50 pre-existing collect failures under vitest (files that
import `bun:test`); passing goes 533 -> 534, unchanged otherwise.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

packages/producer/src/services/render/stages/audioStage.test.ts

@@ -122,7 +122,27 @@ describe("runAudioStage", () => {
     const result = await runAudioStage(makeInput());
     expect(result.hasAudio).toBe(false);
     expect(result.audioError).toMatch(/Audio FX failed for track bgm/);
-    expect(result.audioFailures).toBeUndefined();
+    // And it is classified. This used to come back undefined, so the warning
+    // policy — which reads owner, retryability, reason and stage off this list
+    // — described the FATAL failure with strictly less detail than a single
+    // dropped track gets.
+    expect(result.audioFailures).toEqual([
+      {
+        stage: "internal",
+        reason: "internal",
+        owner: "system",
+        retryable: false,
+        detail: "Audio FX failed for track bgm: browser launch failed",
+      },
+    ]);
+  });
+
+  it("bounds the synthesised failure's detail", async () => {
+    // `detail` is contractually bounded diagnostic text; an ffmpeg-flavoured
+    // message can run to tens of kilobytes.
+    processCompositionAudioMock.mockRejectedValue(new Error("x".repeat(5_000)));
+    const result = await runAudioStage(makeInput());
+    expect(result.audioFailures?.[0]?.detail.length).toBe(2_000);
   });
 
   it("lets an abort keep its own shape rather than becoming an audio error", async () => {

packages/producer/src/services/render/stages/audioStage.ts

@@ -84,12 +84,28 @@ export async function runAudioStage(input: AudioStageInput): Promise<AudioStageR
     } catch (err) {
       // An abort is the caller's own signal and must keep its own shape.
       assertNotAborted();
+      const detail = err instanceof Error ? err.message : String(err);
       return {
         audioOutputPath,
         hasAudio: false,
         audioProcessMs: Date.now() - stage3Start,
-        audioError: err instanceof Error ? err.message : String(err),
-        audioFailures: undefined,
+        audioError: detail,
+        // Synthesised rather than left undefined. The warning policy reads
+        // owner, retryability, reason and stage off this list, so reporting the
+        // FATAL failure — the one that took the whole mix down — with an empty
+        // one gave it strictly less classification than a single dropped track
+        // gets, which is the opposite of what this stage exists to do.
+        // "internal" is the honest bucket: the stages enumerate ffmpeg steps
+        // and this is the FX render, which is none of them.
+        audioFailures: [
+          {
+            stage: "internal",
+            reason: "internal",
+            owner: "system",
+            retryable: false,
+            detail: detail.slice(0, 2_000),
+          },
+        ],
       };
     }
     assertNotAborted();