refactor(app): compose session and player startup

Move streaming, live-session, hydration, local-player, combat, and teleport construction behind the typed Phase-7 boundary. Add exact-owner runtime bindings and focused spawn-claim classification so partial startup rolls back without retaining old session targets while preserving the accepted construction and frame dependencies.

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

src/AcDream.App/Rendering/DevToolsRuntimeSources.cs

@@ -167,6 +167,20 @@ internal sealed class DeferredDevToolsPlayerModeCommands
         _target = target;
     }
 
+    public IDisposable BindOwned(IDevToolsPlayerModeTarget target)
+    {
+        ArgumentNullException.ThrowIfNull(target);
+        ObjectDisposedException.ThrowIf(_deactivated, this);
+        if (_target is not null)
+        {
+            throw new InvalidOperationException(
+                "Developer player-mode commands are already bound.");
+        }
+
+        _target = target;
+        return new Binding(this, target);
+    }
+
     public void Unbind(IDevToolsPlayerModeTarget target)
     {
         ArgumentNullException.ThrowIfNull(target);
@@ -185,6 +199,23 @@ internal sealed class DeferredDevToolsPlayerModeCommands
         if (!_deactivated)
             _target?.ToggleFlyOrChase();
     }
+
+    private sealed class Binding : IDisposable
+    {
+        private DeferredDevToolsPlayerModeCommands? _owner;
+        private readonly IDevToolsPlayerModeTarget _expected;
+
+        public Binding(
+            DeferredDevToolsPlayerModeCommands owner,
+            IDevToolsPlayerModeTarget expected)
+        {
+            _owner = owner;
+            _expected = expected;
+        }
+
+        public void Dispose() =>
+            Interlocked.Exchange(ref _owner, null)?.Unbind(_expected);
+    }
 }
 
 internal interface IDevToolsRuntimeFacts