feat: add publisher and browser client SDK (#9)

Closes #9

src/FinalFactory.Rendezvous.Client/README.md

@@ -0,0 +1,61 @@
+# FinalFactory.Rendezvous.Client
+
+Godot-independent .NET publisher and session-browser SDK for Rendezvous v1.
+The package targets `netstandard2.1` and uses a caller-owned `HttpClient`.
+
+```csharp
+using FinalFactory.Rendezvous.Client;
+using FinalFactory.Rendezvous.Contracts;
+
+using HttpClient http = new()
+{
+    BaseAddress = new Uri("https://rendezvous.example/"),
+};
+
+string publisherCredential = Environment.GetEnvironmentVariable(
+    "RENDEZVOUS_PUBLISHER_CREDENTIAL")
+    ?? throw new InvalidOperationException("Publisher credential is not configured.");
+CancellationToken cancellationToken = default;
+RendezvousPublisherClient publisher = new(http);
+RendezvousClientResult<PublishedSession> registered = await publisher.RegisterAsync(
+    new RegisterSessionRequest
+    {
+        IdempotencyKey = Guid.NewGuid().ToString("N"),
+        GameId = new("space-game"),
+        EnvironmentId = new("production"),
+        RegionId = new("eu-central"),
+        ProtocolVersion = 7,
+        BuildVersion = "1.0.0",
+        DisplayName = "My server",
+        Visibility = ListingVisibility.Public,
+        Capacity = new() { CurrentPlayers = 1, MaximumPlayers = 8 },
+    },
+    publisherCredential,
+    cancellationToken);
+if (!registered.IsSuccess || registered.Value is null)
+{
+    throw new InvalidOperationException(
+        $"Registration failed: {registered.Error} ({registered.Message})");
+}
+```
+
+Load `publisherCredential` from the game's deployment secret boundary; never
+embed it in a client build or source control. A successful registration returns a
+`PublishedSession` containing the lease and host-presence capabilities.
+
+Lease renewal is explicit and caller-controlled:
+
+```csharp
+PublishedSession session = registered.Value;
+await using SessionLeaseMaintainer maintainer = publisher.CreateLeaseMaintainer(
+    session,
+    publisherCredential);
+LeaseMaintenanceResult stopped = await maintainer.RunAsync(cancellationToken);
+```
+
+Creating the maintainer does not start background work. Await its run and dispose
+it when hosting stops. Use `IRendezvousPublisherClient` and
+`IRendezvousSessionBrowserClient` as injection seams in game tests. The SDK disposes
+the requests and responses it creates but never disposes the supplied `HttpClient`.
+
+See the repository's ADR 0007 for retry, paging, ownership, and failure semantics.