DIAG(spawn): emit trace to fd2 (+/scheme/debug); debug: scheme path was wrong

src/platform/redox/mod.rs

@@ -1265,7 +1265,10 @@ impl Pal for Sys {
     ) -> Result<pid_t> {
         use crate::header::spawn::Flags;
         let dbg = |m: &str| {
-            if let Ok(g) = FdGuard::open("debug:", syscall::O_WRONLY) {
+            // fd 2 (login's stderr -> pty -> fbcond -> serial) is proven to
+            // reach the capture; also try the kernel debug scheme directly.
+            let _ = syscall::write(2, m.as_bytes());
+            if let Ok(g) = FdGuard::open("/scheme/debug", syscall::O_WRONLY) {
                 let _ = syscall::write(g.as_raw_fd(), m.as_bytes());
             }
         };