fix(producer): audioPadTrim FFmpeg-8.x-compatible apad invocation

The `audioPadTrim` module's pad-concat step generates a concat script
whose file directives use `file://` URLs (built via Node's
`pathToFileURL`). FFmpeg 8.x on Windows rejects these with
"Impossible to open file:///C:/…" — its `file:` protocol handler strips
the scheme leaving `///C:/…`, which Windows path parsing then rejects.

Field-signal (4 reports over ~24h, all win32/x64, CLI 0.7.59):
  - ts=1784169914 (Baoyu, 60s render, native audio assembly failed)
  - ts=1784177061 (andre 22cores, 345.87s composition, 9 WAV audio elements)
  - ts=1784177375 (KEY DIAGNOSTIC: 13 mono 44.1kHz mp3 tracks, ffmpeg
    8.1.1-full_build gyan.dev, "same project rendered fine in July with
    an older ffmpeg"; manual `ffmpeg -i track.mp3 -af apad=whole_dur=16
    -t 16 -c:a aac out.aac` works with the same binary, so the tool's
    audioPadTrim invocation is the incompatible part)
  - ts=1784177375 (duplicate reporter follow-up)

The concat approach itself is fine — the sibling concat scripts in
`assemble.ts` and `chunkEncoder.ts` pass raw paths (no `pathToFileURL`)
and work on Windows. `audioPadTrim.ts` was the outlier introduced in
PR #1615 (2026-06-20). Aligns with the codebase convention.

Regression pin: unit test asserts the pad-concat stdin never contains
the `file://` scheme, including for a Windows-shaped input path.

End-to-end verification requires a Windows + FFmpeg 8.x reviewer; the
unit test snapshots the arg shape.

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

🤖 Generated with [Claude Code](https://claude.com/claude-code)

— Via

packages/producer/src/services/render/audioPadTrim.test.ts

@@ -47,8 +47,13 @@ describe("buildPadTrimAudioArgs", () => {
     expect(concatArgs[concatArgs.indexOf("-i") + 1]).toBe("pipe:0");
     expect(concatArgs[concatArgs.indexOf("-c:a") + 1]).toBe("copy");
     expect(concatArgs[concatArgs.length - 1]).toBe("/tmp/out.aac");
-    expect(plan.steps[1]!.stdin).toContain("file 'file:///tmp/in.aac'");
-    expect(plan.steps[1]!.stdin).toContain("file 'file:///tmp/out.aac.pad-silence.aac'");
+    // Concat script MUST use bare paths, NOT `file://` URLs. FFmpeg 8.x
+    // on Windows can't open `file:///C:/…` URLs from the concat demuxer
+    // (field-signal ts=1784169914 / 1784177061 / 1784177375). Regression
+    // pin: the `file://` scheme prefix must never appear in the stdin.
+    expect(plan.steps[1]!.stdin).toContain("file '/tmp/in.aac'");
+    expect(plan.steps[1]!.stdin).toContain("file '/tmp/out.aac.pad-silence.aac'");
+    expect(plan.steps[1]!.stdin).not.toContain("file://");
     expect(plan.cleanupPaths).toEqual(["/tmp/out.aac.pad-silence.aac"]);
 
     const reencodedSourceStep = plan.steps.find(
@@ -106,6 +111,31 @@ describe("buildPadTrimAudioArgs", () => {
     const trimNeeded = buildPadTrimAudioArgs("/tmp/a.aac", "/tmp/o.aac", 5.002, 5.0);
     expect(trimNeeded.operation).toBe("trim");
   });
+
+  it("does not emit `file://` URLs in the pad-concat stdin (FFmpeg 8.x Windows compat)", () => {
+    // Regression pin for field-signal reports ts=1784169914 / 1784177061 /
+    // ts=1784177375 (win32/x64, CLI 0.7.59, ffmpeg 8.1.1-full_build). The
+    // concat demuxer's file open on Windows in FFmpeg 8.x rejects
+    // `file:///C:/…` URLs with "Impossible to open …". The concat script
+    // MUST use bare paths. Match sibling `assemble.ts` /
+    // `chunkEncoder.ts` conventions.
+    const winPlan = buildPadTrimAudioPlan(
+      "C:\\Users\\alice\\AppData\\Local\\Temp\\hf-render-abc\\audio.aac",
+      "C:\\Users\\alice\\AppData\\Local\\Temp\\hf-render-abc\\audio-padded.aac",
+      4.0,
+      5.0,
+    );
+    expect(winPlan.operation).toBe("pad");
+    const concatStep = winPlan.steps.find((s) => s.kind === "pad-concat");
+    expect(concatStep).toBeDefined();
+    expect(concatStep!.stdin).toBeDefined();
+    expect(concatStep!.stdin).not.toContain("file://");
+    expect(concatStep!.stdin).not.toContain("file:\\\\");
+    // Bare Windows paths appear as-is in the concat directives.
+    expect(concatStep!.stdin).toContain(
+      "file 'C:\\Users\\alice\\AppData\\Local\\Temp\\hf-render-abc\\audio.aac'",
+    );
+  });
 });
 
 describe("padOrTrimAudioToVideoFrameCount", () => {

packages/producer/src/services/render/audioPadTrim.ts

@@ -20,7 +20,6 @@
 
 import { spawn } from "node:child_process";
 import { rmSync } from "node:fs";
-import { pathToFileURL } from "node:url";
 import {
   extractAudioMetadata,
   formatFfmpegError,
@@ -231,8 +230,17 @@ function channelLayoutForChannels(channels: number | undefined): string {
 }
 
 function concatFileLine(path: string): string {
-  const normalized = pathToFileURL(path).href;
-  return `file '${normalized.replace(/'/g, "'\\''")}'`;
+  // Bare paths in concat directives — NOT `file://` URLs. FFmpeg 8.x on
+  // Windows fails to open URL-form paths from the concat demuxer with
+  // "Impossible to open file:///C:/…" (its file protocol strips the
+  // `file:` prefix leaving `///C:/…`, which Windows path parsing then
+  // rejects). Field-signal reports (ts=1784169914 / 1784177061 /
+  // 1784177375, all win32/x64 CLI 0.7.59; the last isolated the module's
+  // arg shape vs a working manual `apad=whole_dur` command). Bare paths
+  // also match the convention already used by the sibling concat
+  // scripts in `assemble.ts` and `chunkEncoder.ts`. The single-quote
+  // escaping (`'\''`) is the concat demuxer's own escape rule.
+  return `file '${path.replace(/'/g, "'\\''")}'`;
 }
 
 /**