fix: keep stdout pure JSON by redirecting result.Message to stderr for non-default formats

[Copilot]

When --format json is used and the solver returns unsat, the human-readable result.Message was unconditionally printed to stdout before the JSON output, producing mixed content that breaks any JSON consumer.

Change

main.go — mirrors the existing warnings suppression pattern for result.Message:

// Before: message always went to stdout, contaminating JSON output
if formatTmpl == "default" {
    for _, w := range result.Warnings {
        fmt.Fprintln(os.Stderr, w)
    }
}
if result.Message != "" {
    fmt.Println(result.Message)
}

// After: message follows warnings — stdout only for default, stderr for structured formats
if formatTmpl == "default" {
    for _, w := range result.Warnings {
        fmt.Fprintln(os.Stderr, w)
    }
    if result.Message != "" {
        fmt.Println(result.Message)
    }
} else if result.Message != "" {
    fmt.Fprintln(os.Stderr, result.Message)
}

For the default (human-readable) format, behaviour is unchanged. For json and any custom template format, stdout is now pure structured output.

• Fixes Json format template includes other text #70