Refactor duplicate error-checking pattern in wasm compile function

[Copilot]

Addresses code review feedback from #4 to eliminate duplicate error-checking logic in wasm/wasm.go.

Changes

• Extracted processAlerts helper function - Consolidates logic for checking alerts, detecting errors, formatting messages, and either returning errors or accumulating warnings
• Replaced three identical 15-line blocks with single-line calls to processAlerts after lexer, parser, and walker stages
• Used errors.New for error creation - Avoids potential format specifier issues when wrapping pre-formatted alert strings

Before/After

// Before: Repeated 3 times
if len(l.GetAlerts()) > 0 {
    hasError := false
    for _, a := range l.GetAlerts() {
        if a.AlertType() == alerts.Error {
            hasError = true
            break
        }
    }
    msg := formatAlerts(l.GetAlerts(), code)
    if hasError {
        return "", fmt.Errorf("%s", msg)
    }
    warnings.WriteString(msg)
}

// After: Single helper function, called 3 times
if err := processAlerts(l.GetAlerts(), code, &warnings); err != nil {
    return "", err
}

Net reduction: ~42 lines

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