fix(ffmpeg): re-pin dead BtbN tag (first real recording 404'd), wrap download failures in actionable IOException

The 2026-08-09 pin was a DAILY build; BtbN retention aged it out and the cold-cache
download 404'd on the creator's first native recording attempt (2026-09-01,
startup.log). Re-pinned to the MONTH-END build autobuild-2026-08-31-13-27
(N-126342, lgpl-shared win64 — 2-year retention; tag+variant recorded in TASKS.md
per the licensing rule). Verified alive: HEAD 200 + zip contents (bin/ffmpeg.exe,
bin/ffprobe.exe, 7 libav DLLs) match the name-agnostic extractor.
Also closes the wrap-gap: HttpRequestException escaped the locator untouched,
surfacing a raw 'Response status code... 404' from the frame pump; now wrapped in
IOException with the refresh-pin-or-install-ffmpeg message. FfmpegLocatorTests 7/7
(one updated, one added for the 404 case).

ytLive.Tests/FfmpegLocatorTests.cs

@@ -164,6 +164,23 @@ public class FfmpegLocatorTests
     {
         var downloader = new RecordingDownloader { Error = new HttpRequestException("offline") };
         var locator = new FfmpegLocator([], TempDir(), downloader.DownloadAsync);
-        await Assert.ThrowsAsync<HttpRequestException>(() => locator.LocateAsync());
+        var ex = await Assert.ThrowsAsync<IOException>(() => locator.LocateAsync());
+        Assert.IsType<HttpRequestException>(ex.InnerException);
+        Assert.Contains("pinned build", ex.Message, StringComparison.OrdinalIgnoreCase);
+    }
+
+    [Fact]
+    public async Task Locate_DeadPin404_ThrowsActionableLocatorError()
+    {
+        // 2026-09-01 incident: the pinned autobuild tag aged out of BtbN retention;
+        // the raw HttpRequestException must never surface from the pump again.
+        var downloader = new RecordingDownloader
+        {
+            Error = new HttpRequestException("Response status code does not indicate success: 404 (Not Found)."),
+        };
+        var locator = new FfmpegLocator([], TempDir(), downloader.DownloadAsync);
+        var ex = await Assert.ThrowsAsync<IOException>(() => locator.LocateAsync());
+        Assert.Contains("PATH", ex.Message);
+        Assert.Contains("PinnedUrl", ex.Message);
     }
 }