Unique session/client tracking using HyperLogLog (#884)

## Summary

Adds probabilistic cardinality estimation for tracking unique HTTP clients and sessions using a HyperLogLog implementation.

- Add a `HyperLogLog<Precision>` template in `zentelemetry` with thread-safe lock-free register updates, merge support, and XXH3 hashing
- Feed client IP addresses (via raw bytes) and session IDs (via `Oid` bytes) into their respective HyperLogLog estimators from both the ASIO and http.sys server backends
- Emit `distinct_clients` and `distinct_sessions` cardinality estimates in HTTP `CollectStats()`
- Add tests covering empty, single, duplicates, accuracy, merge, and clear scenarios

## Why HyperLogLog

Tracking exact unique counts would require storing every observed IP or session ID. HyperLogLog provides a memory-bounded probabilistic estimate (~1–2% error) using only a few KB of memory regardless of traffic volume.

1 files changed, 17 insertions, 0 deletions

diff --git a/src/zenhttp/servers/httpsys.cpp b/src/zenhttp/servers/httpsys.cpp
index 9fe9a2254..2cad97725 100644
--- a/src/zenhttp/servers/httpsys.cpp
+++ b/src/zenhttp/servers/httpsys.cpp
@@ -2020,6 +2020,23 @@ HttpSysTransaction::InvokeRequestHandler(HttpService& Service, IoBuffer Payload)
 
 	m_HttpServer.MarkRequest();
 
+	// Track distinct client addresses
+	{
+		const SOCKADDR* SockAddr = HttpRequest()->Address.pRemoteAddress;
+		if (SockAddr->sa_family == AF_INET)
+		{
+			const SOCKADDR_IN* V4 = reinterpret_cast<const SOCKADDR_IN*>(SockAddr);
+			m_HttpServer.MarkClientAddress(&V4->sin_addr, sizeof(V4->sin_addr));
+		}
+		else if (SockAddr->sa_family == AF_INET6)
+		{
+			const SOCKADDR_IN6* V6 = reinterpret_cast<const SOCKADDR_IN6*>(SockAddr);
+			m_HttpServer.MarkClientAddress(&V6->sin6_addr, sizeof(V6->sin6_addr));
+		}
+	}
+
+	m_HttpServer.MarkSessionId(ThisRequest.SessionId());
+
 	// Default request handling
 
 #	if ZEN_WITH_OTEL