- Fix: LruCache::get type ambiguity in stats/mod.rs
- Changed `self.cache.get(&key.into())` to `self.cache.get(key)` (key is already &[u8], resolved via Box<[u8]>: Borrow<[u8]>)
- Changed `self.cache.peek(&key)` / `.pop(&key)` to `.peek(key.as_ref())` / `.pop(key.as_ref())` (explicit &[u8] instead of &Box<[u8]>)
- Startup DC ping with RTT display and improved health-check (all DCs, RTT tracking, EMA latency, 30s interval):
- Implemented `LatencyEma` – exponential moving average (α=0.3) for RTT
- `connect()` – measures RTT of each real connection and updates EMA
- `ping_all_dcs()` – pings all 5 DCs via each upstream, returns `Vec<StartupPingResult>` with RTT or error
- `run_health_checks(prefer_ipv6)` – accepts IPv6 preference parameter, rotates DC between cycles (DC1→DC2→...→DC5→DC1...), interval reduced to 30s from 60s, failed checks now mark upstream as unhealthy after 3 consecutive fails
- `DcPingResult` / `StartupPingResult` – public structures for display
- DC Ping at startup: calls `upstream_manager.ping_all_dcs()` before accept loop, outputs table via `println!` (always visible)
- Health checks with `prefer_ipv6`: `run_health_checks(prefer_ipv6)` receives the parameter
- Exported `StartupPingResult` and `DcPingResult`
- Summary: Startup DC ping with RTT, rotational health-check with EMA latency tracking, 30-second interval, correct unhealthy marking after 3 fails.
Co-Authored-By: brekotis <93345790+brekotis@users.noreply.github.com>
