fix(relay): scope manifest classification to .m3u8/.mpd extensions

The relay's "manifest" URL convention (/play/<id>/manifest.<ext>) covers
both HLS playlists and progressive MP4 responses. LooksLikeManifestPath
was matching any filename starting with "manifest.", so manifest.mp4
went through the line-by-line text rewriter and shipped as
Transfer-Encoding: chunked without Content-Length -- WMF/NSPlayer
disconnects on the first byte under that response shape, which surfaced
as immediate load_failure for any YouTube video that resolved to a
progressive MP4 format.

Tighten the heuristic to require either an extensionless "manifest"
filename or a known manifest extension (.m3u8 / .mpd). The Content-Type
fallback still catches manifests served from unconventional URL shapes.

Author: RealWhyKnot

src/WKVRCProxy.Tests/LocalRelayServerTests.cs

@@ -360,9 +360,28 @@ await LocalRelayManifestLocalizer.LocalizeStreamAsync(
     }
 
     [Theory]
+    // Plain m3u8 / mpd extensions always classify as manifests.
     [InlineData("/play/a/manifest.m3u8", "https://node1.whyknot.dev/api/proxy/manifest.m3u8?q=abc", "text/plain", true)]
+    [InlineData("/play/a/index.m3u8", "https://node1.whyknot.dev/api/proxy/index.m3u8", "application/vnd.apple.mpegurl", true)]
+    [InlineData("/play/a/manifest.mpd", "https://node1.whyknot.dev/api/proxy/manifest.mpd", "application/dash+xml", true)]
+    // Query strings don't break the extension probe.
+    [InlineData("/play/a/manifest.m3u8?token=abc", "https://node1.whyknot.dev/api/proxy/manifest.m3u8?token=abc", "application/vnd.apple.mpegurl", true)]
+    // Local path has a non-manifest extension; targetUrl carries .m3u8 -- second branch catches it.
     [InlineData("/play/a/manifest.bin", "https://node1.whyknot.dev/api/proxy/manifest.m3u8?q=abc", "application/octet-stream", true)]
+    // Bare "manifest" (no extension) is treated as a manifest -- some providers ship that.
+    [InlineData("/play/a/manifest", "https://node1.whyknot.dev/api/proxy/manifest", "application/vnd.apple.mpegurl", true)]
+    // /manifest.mp4 was the 2026-05-22 YouTube load_failure bug: the
+    // relay's own progressive-MP4 URL pattern was treated as an HLS
+    // manifest, run through the line-by-line text rewriter, and shipped
+    // as Transfer-Encoding: chunked with no Content-Length, so WMF/NSPlayer
+    // disconnected on the first byte.
+    [InlineData("/play/a/manifest.mp4", "https://node1.whyknot.dev/api/proxy/manifest.mp4?q=abc", "video/mp4", false)]
+    [InlineData("/play/a/MANIFEST.MP4", "https://node1.whyknot.dev/api/proxy/manifest.mp4?q=abc", "video/mp4", false)]
+    // Segment URLs never qualify as manifests, regardless of "manifest" appearing in path.
     [InlineData("/play/a/seg.ts", "https://node1.whyknot.dev/api/proxy/seg.ts?url=abc", "video/mp2t", false)]
+    [InlineData("/play/a/manifest_archive/seg.ts", "https://node1.whyknot.dev/api/proxy/seg.ts", "video/mp2t", false)]
+    // Content-type fallback: a .ts URL that's actually an m3u8 (Tubi pattern) still classifies.
+    [InlineData("/play/a/foo.ts", "https://example.test/playlist.ts", "application/vnd.apple.mpegurl", true)]
     public void ManifestLocalizer_DetectsOnlyManifestShapes(
         string localPath,
         string targetUrl,

src/WKVRCProxy/LocalRelayManifestLocalizer.cs

@@ -201,9 +201,18 @@ private static bool HasManifestExtension(string path)
     private static bool LooksLikeManifestPath(string path)
     {
         string fileName = Path.GetFileName(path);
-        return fileName.StartsWith("manifest.", StringComparison.OrdinalIgnoreCase)
-            || fileName.Equals("manifest", StringComparison.OrdinalIgnoreCase)
-            || path.Contains("/manifest.", StringComparison.OrdinalIgnoreCase);
+        if (fileName.Equals("manifest", StringComparison.OrdinalIgnoreCase))
+            return true;
+        // A "manifest.<ext>" filename is only a manifest when <ext> is one of
+        // the known manifest extensions. The relay also constructs
+        // /play/<id>/manifest.mp4 for progressive MP4 responses, and the
+        // previous unconditional StartsWith branch swept those into the
+        // streaming text rewriter -- the response then went out as
+        // Transfer-Encoding: chunked with no Content-Length and AVPro/WMF
+        // disconnected on the first byte without playing anything.
+        if (fileName.StartsWith("manifest.", StringComparison.OrdinalIgnoreCase))
+            return HasManifestExtension(fileName);
+        return false;
     }
 
     private static bool TryGetUriPath(string url, out string path)