feat(gui): list connected viewers and let the host kick them

Track viewers by endpoint id instead of a bare count. The JSON event
stream gains viewer_joined / viewer_left (each carrying the id),
replacing viewer_count; active/max still ride along so the count
display is unchanged.

The host screen now renders one row per connected viewer with a Kick
button. Clicking it sends `kick <id>` to the headless child over a new
stdin command channel, which the host turns into a per-viewer
CancellationToken cancel; the existing teardown path then emits the
leave, so a kick and a self-disconnect look identical downstream.

The stdin channel only runs under --output json (the GUI shell-out) and
on a detached OS thread, so a read parked on stdin can't hold up the
host's Ctrl+C shutdown.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/gui/child.rs

@@ -6,8 +6,8 @@
 //! egui app drains each frame. stderr is captured into a small ring so a
 //! failed launch (e.g. a missing gst plugin) can be surfaced in the window.
 
-use std::io::{BufRead, BufReader};
-use std::process::{Child, Command, Stdio};
+use std::io::{BufRead, BufReader, Write};
+use std::process::{Child, ChildStdin, Command, Stdio};
 use std::sync::mpsc::Receiver;
 use std::sync::{Arc, Mutex};
 use std::time::Duration;
@@ -35,7 +35,13 @@ pub enum ChildEvent {
         max_viewers: u32,
         max_viewers_source: String,
     },
-    ViewerCount {
+    ViewerJoined {
+        id: String,
+        active: u32,
+        max: u32,
+    },
+    ViewerLeft {
+        id: String,
         active: u32,
         max: u32,
     },
@@ -63,6 +69,9 @@ pub struct ChildProc {
     child: Child,
     pub rx: Receiver<ChildEvent>,
     stderr_tail: Arc<Mutex<Vec<String>>>,
+    /// Write end of the child's stdin, for the line-based command channel
+    /// (see [`ChildProc::send_command`]). `None` once it's been closed.
+    stdin: Option<ChildStdin>,
 }
 
 impl ChildProc {
@@ -72,11 +81,14 @@ impl ChildProc {
         let exe = std::env::current_exe()?;
         let mut child = Command::new(exe)
             .args(args)
-            .stdin(Stdio::null())
+            // Piped so we can send line commands (e.g. `kick <id>`); the host
+            // only reads it when driven this way (`--output json`).
+            .stdin(Stdio::piped())
             .stdout(Stdio::piped())
             .stderr(Stdio::piped())
             .spawn()?;
 
+        let stdin = child.stdin.take();
         let (tx, rx) = std::sync::mpsc::channel();
         let stdout = child.stdout.take().expect("stdout piped");
         std::thread::spawn(move || {
@@ -114,9 +126,23 @@ impl ChildProc {
             child,
             rx,
             stderr_tail,
+            stdin,
         })
     }
 
+    /// Send one newline-terminated command to the child over its stdin (the
+    /// host parses these as `kick <endpoint-id>`). Best-effort: a closed pipe
+    /// (child already gone) just drops the command.
+    pub fn send_command(&mut self, cmd: &str) {
+        let Some(stdin) = self.stdin.as_mut() else {
+            return;
+        };
+        if let Err(e) = writeln!(stdin, "{cmd}") {
+            tracing::warn!("failed to send command to host child: {e}");
+            self.stdin = None; // pipe is dead; stop trying
+        }
+    }
+
     /// Whether the child is still running.
     pub fn is_alive(&mut self) -> bool {
         matches!(self.child.try_wait(), Ok(None))
@@ -205,10 +231,14 @@ mod tests {
     }
 
     #[test]
-    fn viewer_count_round_trips() {
+    fn viewer_join_leave_round_trip() {
         assert!(matches!(
-            parse(Event::ViewerCount { active: 2, max: 4 }),
-            ChildEvent::ViewerCount { active: 2, max: 4 }
+            parse(Event::ViewerJoined { id: "nodeXYZ", active: 2, max: 4 }),
+            ChildEvent::ViewerJoined { id, active: 2, max: 4 } if id == "nodeXYZ"
+        ));
+        assert!(matches!(
+            parse(Event::ViewerLeft { id: "nodeXYZ", active: 1, max: 4 }),
+            ChildEvent::ViewerLeft { id, active: 1, max: 4 } if id == "nodeXYZ"
         ));
     }