brush: route rb_dbg diagnostics to /scheme/debug instead of stderr

Writing startup diagnostics to stderr interleaves with the interactive
console on the live image; /scheme/debug reaches the serial log without
disturbing the shell's tty.

local/recipes/shells/brush/patches/brush-redox-runtime-and-input.patch

@@ -78,8 +78,8 @@ index 15340ad..46774c3 100644
 +#[doc(hidden)]
 +fn rb_dbg(m: &str) {
 +    use std::io::Write;
-+    let _ = std::io::stderr().write_all(m.as_bytes());
-+    let _ = std::io::stderr().flush();
++    let df = std::fs::OpenOptions::new().write(true).open("/scheme/debug");
++    if let Ok(mut df) = df { let _ = df.write_all(m.as_bytes()); }
 +}
 +
  /// Main entry point for the `brush` shell.