v2.5.6: CosmoKvHttp client supports Endpoint=unix:///path UDS connections

Co-located clients (mail-server containers, CLI tools running on the
same host as cosmokvd) can now bypass TCP loopback by setting their
connection string to `Endpoint=unix:///var/run/cosmokvd.sock;...`.
Saves the per-call socket-buffer copies + TCP framing — typically
20-40% lower latency on local IPC.

Implementation: CosmoKvHttpConnection inspects the Endpoint prefix at
construction time. If it starts with `unix://`, a SocketsHttpHandler
with a ConnectCallback is wired up, returning a NetworkStream over a
freshly-connected UnixDomainSocketEndPoint. HttpClient gets a sentinel
`http://cosmokvd-uds/` BaseAddress (the host portion is irrelevant
because ConnectCallback short-circuits DNS/TCP).

Backwards compatible — TCP endpoints work exactly as before.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: vkuttyp

Directory.Build.props

@@ -7,6 +7,6 @@
     <Authors>vkuttyp</Authors>
     <PackageLicenseExpression>MIT</PackageLicenseExpression>
     <RepositoryUrl>https://github.com/vkuttyp/CosmoSQLClient-Dotnet</RepositoryUrl>
-    <Version>2.5.5</Version>
+    <Version>2.5.6</Version>
   </PropertyGroup>
 </Project>

src/CosmoSQLClient.CosmoKvHttp/CosmoKvHttpConnection.cs

@@ -1,6 +1,7 @@
 using System.Net;
 using System.Net.Http.Headers;
 using System.Net.Http.Json;
+using System.Net.Sockets;
 using System.Runtime.CompilerServices;
 using System.Text.Json;
 using CosmoSQLClient.Core;
@@ -47,14 +48,51 @@ public CosmoKvHttpConnection(CosmoKvHttpConfiguration config, HttpClient http)
     public CosmoKvHttpConnection(CosmoKvHttpConfiguration config)
     {
         _config = config;
-        _http   = new HttpClient
+        _http   = BuildHttpClient(config);
+        _http.DefaultRequestHeaders.Authorization =
+            new AuthenticationHeaderValue("Bearer", config.AuthToken);
+        _ownsHttp = true;
+    }
+
+    /// <summary>
+    /// Build the underlying <see cref="HttpClient"/>. When
+    /// <c>config.Endpoint</c> starts with <c>unix://</c> we route the
+    /// connection over a Unix Domain Socket instead of TCP — saves the
+    /// per-call loopback overhead when cosmokvd runs on the same host
+    /// (the typical co-located mail-server deployment). The path after
+    /// <c>unix://</c> is the absolute socket path on disk. The HTTP
+    /// virtual host portion of requests becomes irrelevant since
+    /// <see cref="SocketsHttpHandler.ConnectCallback"/> short-circuits
+    /// the DNS/TCP machinery — we use a sentinel <c>cosmokvd-uds</c>
+    /// host so any logging is recognisable.
+    /// </summary>
+    private static HttpClient BuildHttpClient(CosmoKvHttpConfiguration config)
+    {
+        const string UnixPrefix = "unix://";
+        if (config.Endpoint.StartsWith(UnixPrefix, StringComparison.OrdinalIgnoreCase))
+        {
+            string socketPath = config.Endpoint.Substring(UnixPrefix.Length);
+            var handler = new SocketsHttpHandler
+            {
+                ConnectCallback = async (ctx, ct) =>
+                {
+                    var ep = new UnixDomainSocketEndPoint(socketPath);
+                    var sock = new Socket(AddressFamily.Unix, SocketType.Stream, ProtocolType.Unspecified);
+                    await sock.ConnectAsync(ep, ct).ConfigureAwait(false);
+                    return new NetworkStream(sock, ownsSocket: true);
+                },
+            };
+            return new HttpClient(handler)
+            {
+                BaseAddress = new Uri("http://cosmokvd-uds/"),
+                Timeout     = config.Timeout,
+            };
+        }
+        return new HttpClient
         {
             BaseAddress = new Uri(config.Endpoint),
             Timeout     = config.Timeout,
         };
-        _http.DefaultRequestHeaders.Authorization =
-            new AuthenticationHeaderValue("Bearer", config.AuthToken);
-        _ownsHttp = true;
     }
 
     public CosmoKvHttpConnection(string connectionString)