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>

tests/AcDream.App.Tests/Composition/SessionStartCompositionTests.cs

@@ -0,0 +1,38 @@
+using AcDream.App.Composition;
+using AcDream.App.Net;
+
+namespace AcDream.App.Tests.Composition;
+
+public sealed class SessionStartCompositionTests
+{
+    [Fact]
+    public void DiagnosticsPreserveMissingCredentialAndFailureMessagesOnly()
+    {
+        var messages = new List<string>();
+
+        SessionStartCompositionPhase.Report(
+            new LiveSessionStartResult(
+                LiveSessionStartStatus.MissingCredentials),
+            messages.Add);
+        SessionStartCompositionPhase.Report(
+            new LiveSessionStartResult(
+                LiveSessionStartStatus.Failed,
+                Error: new InvalidOperationException("boom")),
+            messages.Add);
+        SessionStartCompositionPhase.Report(
+            new LiveSessionStartResult(LiveSessionStartStatus.Disabled),
+            messages.Add);
+        SessionStartCompositionPhase.Report(
+            new LiveSessionStartResult(LiveSessionStartStatus.Connected),
+            messages.Add);
+
+        Assert.Equal(2, messages.Count);
+        Assert.Equal(
+            "live: ACDREAM_LIVE set but TEST_USER/TEST_PASS missing; skipping",
+            messages[0]);
+        Assert.StartsWith(
+            "live: session failed: System.InvalidOperationException: boom",
+            messages[1],
+            StringComparison.Ordinal);
+    }
+}