Macro retry 

macro_rules! retry {
    (attempts: $max:expr, backoff: $backoff:expr, $operation:expr) => { ... };
    ($max:expr, $operation:expr) => { ... };
}

Retries an operation with configurable backoff.

Semantics

let result = retry!(
    attempts: 3,
    backoff: exponential(100ms, 2.0),
    operation()
).await;

• Retries up to max_attempts times
• Waits delay between attempts (optionally with exponential backoff)
• Returns first success, or last error after exhausting retries
• Respects cancellation during both operation and delay