fix(appd): drain module stdout after READY signal to prevent pipe stall

wait_for_ready() now returns the BufReader<ChildStdout> with the READY
line already consumed. supervise() spawns drain_stdout() on that reader
so any subsequent module output is forwarded to the trace log and the
pipe buffer never fills up.

Without this, a long-running Wasm module that writes to stdout after
printing READY would eventually block waiting on a full pipe.

crates/weft-appd/src/runtime.rs

@@ -37,7 +37,7 @@ pub(crate) async fn supervise(
     let ready_result = tokio::time::timeout(READY_TIMEOUT, wait_for_ready(stdout)).await;
 
     match ready_result {
-        Ok(Ok(())) => {
+        Ok(Ok(remaining_stdout)) => {
             registry
                 .lock()
                 .await
@@ -47,6 +47,7 @@ pub(crate) async fn supervise(
                 app_id: app_id.to_owned(),
             });
             tracing::info!(session_id, %app_id, "app ready");
+            tokio::spawn(drain_stdout(remaining_stdout, session_id));
         }
         Ok(Err(e)) => {
             tracing::warn!(session_id, %app_id, error = %e, "stdout read error before READY");
@@ -82,14 +83,31 @@ pub(crate) async fn supervise(
     Ok(())
 }
 
-async fn wait_for_ready(stdout: tokio::process::ChildStdout) -> anyhow::Result<()> {
-    let mut lines = BufReader::new(stdout).lines();
-    while let Some(line) = lines.next_line().await? {
+async fn wait_for_ready(
+    stdout: tokio::process::ChildStdout,
+) -> anyhow::Result<BufReader<tokio::process::ChildStdout>> {
+    let mut reader = BufReader::new(stdout);
+    loop {
+        let mut line = String::new();
+        let n = reader.read_line(&mut line).await?;
+        if n == 0 {
+            return Err(anyhow::anyhow!("stdout closed without READY signal"));
+        }
         if line.trim() == "READY" {
-            return Ok(());
+            return Ok(reader);
+        }
+    }
+}
+
+async fn drain_stdout(mut reader: BufReader<tokio::process::ChildStdout>, session_id: u64) {
+    let mut line = String::new();
+    loop {
+        line.clear();
+        match reader.read_line(&mut line).await {
+            Ok(0) | Err(_) => break,
+            Ok(_) => tracing::debug!(session_id, stdout = %line.trim_end(), "app stdout"),
         }
     }
-    Err(anyhow::anyhow!("stdout closed without READY signal"))
 }
 
 async fn drain_stderr(stderr: tokio::process::ChildStderr, session_id: u64) {