- proxy_config on CrawlerRunConfig now accepts a single ProxyConfig or
a list of ProxyConfig tried in order (first-come-first-served)
- Remove is_fallback from ProxyConfig and fallback_proxy_configs from
CrawlerRunConfig — proxy escalation handled entirely by list order
- Add _get_proxy_list() normalizer for the retry loop
- Add CrawlResult.crawl_stats with attempts, retries, proxies_used,
fallback_fetch_used, and resolved_by for billing and observability
- Set success=False with error_message when all attempts are blocked
- Simplify retry loop — no more is_fallback stashing logic
- Update docs and tests to reflect new API
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