Working Wayland end-to-end: gst owns the pipeline, ffmpeg just serves

Moves the full capture+encode+mux pipeline into gst-launch, leaving
ffmpeg as a thin HTTP server. Verified end-to-end on KDE Plasma 6
Wayland: screencast portal → mpv mirror-tunnel rendering in real time.

Pipeline:
  pipewiresrc(do-timestamp) → videoconvert → x264enc (zerolatency
    ultrafast) → h264parse(config-interval=-1) → byte-stream caps →
    mpegtsmux ← (aacparse ← avenc_aac ← audioconvert ← pulsesrc) →
    fdsink fd=1
  ffmpeg -fflags nobuffer+discardcorrupt+genpts -flags low_delay
    -analyzeduration 0 -probesize 32 -f mpegts -i pipe:0 -c copy
    -f mpegts -listen 1 http://127.0.0.1:N

Why each piece is load-bearing (do not relitigate without cause):

- x264enc + h264parse + byte-stream caps: raw video over a pipe hits
  stride/format negotiation problems (green screens with mis-aligned
  rows). Encoding inside gst sidesteps that entirely.
- mpegtsmux inside gst: H.264 Annex B carries no timestamps. Without
  a container, ffmpeg sees "Timestamps are unset" and downstream
  muxing breaks. mpegts in gst preserves pipewiresrc's clock.
- byte-stream + alignment=au caps: h264parse defaults to AVC format
  (length-prefixed NALUs) for some downstreams; ffmpeg's mpegts
  demuxer needs Annex B start codes.
- audio in gst (pulsesrc + avenc_aac): keeping ffmpeg as a pure
  passthrough (`-c copy`) avoids ffmpeg's audio-input dependency
  delaying HTTP serving until both inputs are ready.
- `-analyzeduration 0 -probesize 32`: stop ffmpeg from buffering 5MB
  / 5s of input before deciding it understands the stream.
- Also fixes a separate one-shot bug from earlier: the previous
  health-probe in wait_for_listener consumed ffmpeg's single
  `-listen 1` accept slot, so the actual bridge connect hit
  Connection refused. Replaced with connect_to_capture which
  returns the bridge socket.

Adds dep checks for pipewiresrc, x264enc, h264parse, mpegtsmux,
pulsesrc, avenc_aac, aacparse with per-distro install hints.

Known gap: VLC currently shows a green screen against the stream
even though mpv works fine. Likely VLC-specific demuxer/latency
settings, not a pipeline correctness issue — to investigate as a
follow-up. mpv is the recommended client either way.

src/common/deps.rs

@@ -10,6 +10,12 @@ pub fn check_host_binaries(display: DisplayServer) -> Result<()> {
         require("gst-launch-1.0")?;
         require("gst-inspect-1.0")?;
         require_gst_element("pipewiresrc")?;
+        require_gst_element("x264enc")?;
+        require_gst_element("h264parse")?;
+        require_gst_element("mpegtsmux")?;
+        require_gst_element("pulsesrc")?;
+        require_gst_element("avenc_aac")?;
+        require_gst_element("aacparse")?;
     }
     Ok(())
 }
@@ -72,6 +78,34 @@ fn install_hint_for_gst_element(name: &str) -> String {
             Some("opensuse" | "opensuse-tumbleweed" | "opensuse-leap") => "pipewire-gstreamer",
             _ => "the GStreamer PipeWire plugin",
         },
+        "x264enc" => match distro.as_deref() {
+            Some("arch" | "cachyos" | "manjaro" | "endeavouros") => "gst-plugins-ugly",
+            Some("debian" | "ubuntu" | "pop" | "linuxmint") => "gstreamer1.0-plugins-ugly",
+            Some("fedora" | "nobara") => "gstreamer1-plugins-ugly",
+            Some("opensuse" | "opensuse-tumbleweed" | "opensuse-leap") => "gstreamer-plugins-ugly",
+            _ => "the GStreamer x264 plugin",
+        },
+        "h264parse" | "mpegtsmux" | "aacparse" => match distro.as_deref() {
+            Some("arch" | "cachyos" | "manjaro" | "endeavouros") => "gst-plugins-bad",
+            Some("debian" | "ubuntu" | "pop" | "linuxmint") => "gstreamer1.0-plugins-bad",
+            Some("fedora" | "nobara") => "gstreamer1-plugins-bad-free",
+            Some("opensuse" | "opensuse-tumbleweed" | "opensuse-leap") => "gstreamer-plugins-bad",
+            _ => "the GStreamer plugins-bad set",
+        },
+        "pulsesrc" => match distro.as_deref() {
+            Some("arch" | "cachyos" | "manjaro" | "endeavouros") => "gst-plugins-good",
+            Some("debian" | "ubuntu" | "pop" | "linuxmint") => "gstreamer1.0-pulseaudio",
+            Some("fedora" | "nobara") => "gstreamer1-plugins-good",
+            Some("opensuse" | "opensuse-tumbleweed" | "opensuse-leap") => "gstreamer-plugins-good",
+            _ => "the GStreamer PulseAudio plugin",
+        },
+        "avenc_aac" => match distro.as_deref() {
+            Some("arch" | "cachyos" | "manjaro" | "endeavouros") => "gst-libav",
+            Some("debian" | "ubuntu" | "pop" | "linuxmint") => "gstreamer1.0-libav",
+            Some("fedora" | "nobara") => "gstreamer1-libav",
+            Some("opensuse" | "opensuse-tumbleweed" | "opensuse-leap") => "gstreamer-libav",
+            _ => "the GStreamer libav (avenc) plugin",
+        },
         _ => name,
     };
     install_command(&distro, pkg)