bug(cli): --max-weight threshold error prints twice

[rocketman-code]

Current Behavior

When --max-weight threshold is exceeded, the error message is printed twice: once from the explicit eprintln! + process::exit(1) in run_trace, and again from the main() error handler which catches the non-zero exit.

Expected Behavior

The error should be printed exactly once.

Context

Duplicate error messages are confusing and look like a bug to users.

Technical Details

Relevant Code

src/main.rs:365-384 — run_trace

if let Some(threshold) = args.max_weight.filter(|&t| report.static_weight_bytes > t) {
    eprintln!("error: ... exceeds --max-weight threshold ...");
    std::process::exit(1);
}

The process::exit(1) bypasses the normal return path. However, since run_trace returns Result<(), Error>, it should return an Err variant instead, letting the main() error handler print once and exit.