Implement streaming I/O for external command execution

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Background

The current plan for issue #9 (wire executable I/O through ShellIo) uses Command::output(), which buffers the child process's entire stdout/stderr before writing it to ShellIo. This is correct but not streaming — output is held in memory until the child exits.

For a performant shell, output should be forwarded to the caller incrementally as the child produces it, rather than buffered entirely in memory.

Problem

ShellIo::out_writer() returns &mut dyn Write, and std::process::Command accepts Stdio (file handles, pipes, or inherit) — there is no way to hand a dyn Write directly to the OS-level pipe. Bridging the two requires either:

• A dedicated reader thread per child stream (stdout + stderr), copying bytes from the pipe read end into ShellIo as they arrive
• An async/io-uring approach for non-blocking I/O multiplexing

Acceptance Criteria

• External command stdout and stderr are forwarded to ShellIo incrementally (no full-output buffering)
• MockIo still captures all output correctly in tests
• Works for both StandardIo (real terminal) and MockIo (test harness)
• Large output (e.g., piping a multi-GB file) does not cause excessive memory use
• No regression on existing executable I/O tests

Notes

• Blocked by / should follow Wire executable I/O through ShellIo #9, which establishes the basic ShellIo wiring
• Threading approach is likely simplest for now; async can be revisited later
• stdin forwarding (interactive child processes) is a related but separate concern