refactor(app): make session start terminal

Move live-session startup and its existing diagnostics into the typed Phase-9 owner. GameWindow.OnLoad now ends immediately after that phase, so no callback binding, publication, or allocation can occur after the session becomes active.

Co-authored-by: Codex <codex@openai.com>

src/AcDream.App/Composition/SessionStartComposition.cs

@@ -0,0 +1,47 @@
+using AcDream.App.Net;
+
+namespace AcDream.App.Composition;
+
+internal sealed record SessionStartDependencies(
+    RuntimeOptions Options,
+    Action<string> Log);
+
+/// <summary>
+/// Terminal startup phase. Every callback, command target, and frame root is
+/// already published when this owner asks the canonical session host to start.
+/// </summary>
+internal sealed class SessionStartCompositionPhase
+    : ISessionStartCompositionPhase<FrameRootResult>
+{
+    private readonly SessionStartDependencies _dependencies;
+
+    public SessionStartCompositionPhase(SessionStartDependencies dependencies) =>
+        _dependencies = dependencies
+            ?? throw new ArgumentNullException(nameof(dependencies));
+
+    public void Start(FrameRootResult frame)
+    {
+        ArgumentNullException.ThrowIfNull(frame);
+        LiveSessionStartResult result =
+            frame.SessionHost.Start(_dependencies.Options);
+        Report(result, _dependencies.Log);
+    }
+
+    internal static void Report(
+        LiveSessionStartResult result,
+        Action<string> log)
+    {
+        ArgumentNullException.ThrowIfNull(result);
+        ArgumentNullException.ThrowIfNull(log);
+        switch (result.Status)
+        {
+            case LiveSessionStartStatus.MissingCredentials:
+                log(
+                    "live: ACDREAM_LIVE set but TEST_USER/TEST_PASS missing; skipping");
+                break;
+            case LiveSessionStartStatus.Failed:
+                log($"live: session failed: {result.Error}");
+                break;
+        }
+    }
+}