Normalized retry configuration with all optional fields resolved to concrete values.

Produced by normalizeRuntimeConfig from the user-provided RetryConfig. Guarantees that all fields are present and validated, safe for direct use by retryWithBackoff.

interface NormalizedRetryConfig {
    maxAttempts: number;
    initialDelayMs: number;
    maxDelayMs: number;
    multiplier: number;
    jitter: boolean;
}