Printing anything prevents reading from stdin

Bug report form

• I have done a basic search of the issue tracker to find any existing issues that are similar.
• I have checked that my version is at least the latest stable release available via my installation method.

Describe the bug

It's not possible to write to stdout or stderr and then process input line by line. Consider the following:

def main [] {
    print --stderr "Interpreter listening..."
    lines | each { str reverse | print }    
    print --stderr "Interpreter done."
}

The idea to use lines like this comes from #14901. I've also tried using input in a loop, but I can't figure out how to terminate it on EOF.

How to reproduce

Save the above script as sample.nu. Then run it:

$ nu --stdin sample.nu
Interpreter listening...
Interpreter done.

It exits without waiting for the input. Then comment out the first print statement, run it again and see it working correctly.

Expected behavior

The script should print to stderr, then wait for input. Each input line should be immediately printed in reverse. Pressing ctrl-d (or closing the input stream piped from another process) should result in the closing message being printed. After that the script should terminate without any error.

Configuration

Start by running the reported nu --stdin sample.nu reproducer and compare it with the version where the initial print is removed. Trace the lines and input entry points together with stdout/stderr handling, then verify that input remains readable, lines are processed immediately, EOF is handled cleanly, and the closing message appears.