TASK-032-001: Extend ConnectionPolicy with multi-connection config

Add MaxHttp2ConnectionsPerHost (default 2), MaxHttp3ConnectionsPerHost (default 2),
and SlotQueueSize (default 1, max 1) to ConnectionPolicy. HTTP/1.0 and HTTP/1.1
remain hard-coded to 1 connection per host. SlotQueueSize throws ArgumentOutOfRangeException
if set outside [1..1] to preserve the Offering-based capacity signal for multi-slot routing.

Author: st0o0

.maggus/features/feature_032.md

@@ -41,13 +41,13 @@ Two architectural improvements to TurboHttp's stream pipeline:
 **Parallel:** yes — can run alongside TASK-032-004
 
 **Acceptance Criteria:**
-- [ ] `ConnectionPolicy` gains `MaxHttp2ConnectionsPerHost` (default `2`)
-- [ ] `ConnectionPolicy` gains `MaxHttp3ConnectionsPerHost` (default `2`)
-- [ ] `ConnectionPolicy` gains `SlotQueueSize` (default `1`) — per-slot Source.Queue buffer size; must not exceed `1` to ensure `Offering=true` propagates after at most one queued item
-- [ ] HTTP/1.0 and HTTP/1.1 are hard-coded to 1 connection per host (not configurable via these properties)
-- [ ] XML doc comments explain each property
-- [ ] Existing `ConnectionPolicy` tests still pass
-- [ ] Build succeeds with zero errors
+- [x] `ConnectionPolicy` gains `MaxHttp2ConnectionsPerHost` (default `2`)
+- [x] `ConnectionPolicy` gains `MaxHttp3ConnectionsPerHost` (default `2`)
+- [x] `ConnectionPolicy` gains `SlotQueueSize` (default `1`) — per-slot Source.Queue buffer size; must not exceed `1` to ensure `Offering=true` propagates after at most one queued item
+- [x] HTTP/1.0 and HTTP/1.1 are hard-coded to 1 connection per host (not configurable via these properties)
+- [x] XML doc comments explain each property
+- [x] Existing `ConnectionPolicy` tests still pass
+- [x] Build succeeds with zero errors
 
 ---
 

src/TurboHttp/Protocol/RFC9112/ConnectionPolicy.cs

@@ -22,4 +22,50 @@ public sealed record ConnectionPolicy
     /// Default is true.
     /// </summary>
     public bool AllowHttp2Multiplexing { get; init; } = true;
+
+    /// <summary>
+    /// Maximum number of parallel HTTP/2 connections opened per host.
+    /// When a connection reaches its <c>MaxConcurrentStreams</c> limit, the pipeline
+    /// backpressures and a new connection is opened — up to this limit.
+    /// HTTP/1.0 and HTTP/1.1 always use exactly one connection per host regardless of this value.
+    /// Default is 2.
+    /// </summary>
+    public int MaxHttp2ConnectionsPerHost { get; init; } = 2;
+
+    /// <summary>
+    /// Maximum number of parallel HTTP/3 connections opened per host.
+    /// When a connection reaches its stream limit, the pipeline backpressures
+    /// and a new connection is opened — up to this limit.
+    /// HTTP/1.0 and HTTP/1.1 always use exactly one connection per host regardless of this value.
+    /// Default is 2.
+    /// </summary>
+    public int MaxHttp3ConnectionsPerHost { get; init; } = 2;
+
+    private int _slotQueueSize = 1;
+
+    /// <summary>
+    /// Per-slot <c>Source.Queue</c> buffer size used by the multi-connection routing stage.
+    /// Must be exactly <c>1</c>: a value greater than 1 would allow multiple requests to
+    /// accumulate on a saturated connection slot without triggering the <c>Offering=true</c>
+    /// backpressure signal, defeating the purpose of multi-connection routing.
+    /// Default is 1.
+    /// </summary>
+    /// <exception cref="ArgumentOutOfRangeException">
+    /// Thrown when the value is less than 1 or greater than 1.
+    /// </exception>
+    public int SlotQueueSize
+    {
+        get => _slotQueueSize;
+        init
+        {
+            if (value < 1 || value > 1)
+            {
+                throw new ArgumentOutOfRangeException(
+                    nameof(SlotQueueSize),
+                    value,
+                    "SlotQueueSize must be exactly 1. A value greater than 1 defeats the Offering-based capacity signal used for multi-connection routing.");
+            }
+            _slotQueueSize = value;
+        }
+    }
 }

src/TurboHttp/Transport/IClientProvider.cs

@@ -92,7 +92,7 @@ public ValueTask DisposeAsync()
         return ValueTask.CompletedTask;
     }
 
-    private Socket CreateSocket()
+    private static Socket CreateSocket()
     {
         // On Linux, new Socket(AddressFamily.Unspecified, ...) throws SocketException
         // "Protocol not supported" because AF_UNSPEC + IPPROTO_TCP is invalid.