feat: Add structured retry/backoff engine

[rexlunae]

Summary

Adds a generic retry engine with exponential backoff from @aecs4u (PR #21).

Features

Retry Reasons

Classifies transient failures:

• Connect failures
• Timeouts
• Rate limiting (429)
• Server errors (5xx)
• Request timeouts

Backoff Strategies

• Exponential backoff with jitter
• Configurable base delay, max delay, max attempts
• Respects Retry-After headers from rate-limited responses

Usage

use rustyclaw::retry::{RetryExecutor, RetryPolicy};

let policy = RetryPolicy {
    max_attempts: 3,
    base_delay: Duration::from_secs(1),
    max_delay: Duration::from_secs(30),
    jitter: true,
};

let executor = RetryExecutor::new(policy);
let result = executor.execute(|| async {
    make_api_call().await
}).await;

Files

• src/retry/mod.rs — Core retry executor and types (226 lines)
• src/retry/policy.rs — Configurable retry policies (74 lines)

No New Dependencies

Uses only std::time::Duration and std::future::Future.

Attribution

Original implementation by @aecs4u