debug: add PIXELPASS_TS_DUMP tap for A/V drift analysis

When PIXELPASS_TS_DUMP=<path> is set, tee the muxed MPEG-TS to a file in
addition to the normal fd=1 serve path, so the host-side stream can be
ffprobe'd for capture-side audio/video PTS drift. Each tee branch gets its
own queue so the disk sink cannot backpressure the live serve branch.

No effect when the variable is unset, mirroring PIXELPASS_GST_DEBUG.

Used to establish that the host produces an A/V-clean realtime stream
(+/-18 ms over 170 s), ruling out the capture side in the screen-share
drift investigation.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/host/pipeline.rs

@@ -189,9 +189,32 @@ fn build_args(
         "!".into(),
         "queue".into(),
         "!".into(),
-        "fdsink".into(),
-        "fd=1".into(),
     ];
+    // Debug A/V-drift tap: when PIXELPASS_TS_DUMP=<path> is set, tee the exact
+    // muxed TS both to fd=1 (normal serve path, unchanged) and to a file, so the
+    // host-side stream can be ffprobe'd for capture-side audio/video PTS drift.
+    // Each tee branch has its own queue so the disk sink can't backpressure the
+    // live serve branch. No effect when unset. (Mirrors PIXELPASS_GST_DEBUG.)
+    if let Some(dump) = std::env::var_os("PIXELPASS_TS_DUMP") {
+        let path = dump.to_string_lossy().into_owned();
+        args.extend([
+            "tee".into(),
+            "name=dbgtee".into(),
+            "!".into(),
+            "queue".into(),
+            "!".into(),
+            "fdsink".into(),
+            "fd=1".into(),
+            "dbgtee.".into(),
+            "!".into(),
+            "queue".into(),
+            "!".into(),
+            "filesink".into(),
+            format!("location={path}"),
+        ]);
+    } else {
+        args.extend(["fdsink".into(), "fd=1".into()]);
+    }
 
     // Downscale step for the quality presets. `None` = encode at native size
     // (the "Source" preset, or a source already at/below the target height — we