Vug::LoopState

Mutable state threaded through the fetch loop, replacing scattered local variables and positional tuple returns.

NOT thread-safe. Each Fetcher#fetch call must create its own instance. Do not share across fibers.

Owns the per-request state for a single Fetcher#fetch call: the URL being chased, the redirect/gray-placeholder counters, the visited-set used to detect redirect loops, the initial DNS resolutions used to detect DNS rebinding, and the start time used to enforce the timeout.