refactor(runtime): own canonical action state

Move selection, combat, and interaction target mode under one Runtime owner; make plugins, retained UI, session routing, and typed runtime views borrow its exact children; and add failure-safe reset, instance isolation, source ownership, and normalized checkpoint coverage without changing retail ordering.

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

src/AcDream.Runtime/Gameplay/InteractionState.cs

@@ -0,0 +1,96 @@
+namespace AcDream.Runtime.Gameplay;
+
+public enum InteractionModeKind
+{
+    None,
+    Use,
+    Examine,
+    UseItemOnTarget,
+}
+
+public readonly record struct InteractionMode(
+    InteractionModeKind Kind,
+    uint SourceObjectId = 0)
+{
+    public static InteractionMode None => new(InteractionModeKind.None);
+}
+
+public readonly record struct InteractionModeTransition(
+    InteractionMode Previous,
+    InteractionMode Current);
+
+/// <summary>
+/// Canonical presentation-independent owner for retail's temporary target
+/// mode. App projects this state into a cursor image; it does not own or copy
+/// the mode.
+/// </summary>
+public sealed class InteractionState
+{
+    public InteractionMode Current { get; private set; } = InteractionMode.None;
+
+    public event Action<InteractionModeTransition>? Changed;
+
+    public bool EnterUse() => Set(new InteractionMode(InteractionModeKind.Use));
+
+    public bool EnterExamine() =>
+        Set(new InteractionMode(InteractionModeKind.Examine));
+
+    public bool EnterUseItemOnTarget(uint sourceObjectId)
+    {
+        if (sourceObjectId == 0)
+            throw new ArgumentOutOfRangeException(nameof(sourceObjectId));
+        return Set(new InteractionMode(
+            InteractionModeKind.UseItemOnTarget,
+            sourceObjectId));
+    }
+
+    public bool Clear() => Set(InteractionMode.None);
+
+    /// <summary>
+    /// Publishes the session-reset edge even when the mode is already clear so
+    /// a retry can repair any observer that missed an earlier notification.
+    /// State commits before observers run and every observer is attempted.
+    /// </summary>
+    public void ResetSession()
+    {
+        InteractionMode previous = Current;
+        Current = InteractionMode.None;
+        Action<InteractionModeTransition>? listeners = Changed;
+        if (listeners is null)
+            return;
+
+        var transition = new InteractionModeTransition(
+            previous,
+            InteractionMode.None);
+        List<Exception>? failures = null;
+        foreach (Action<InteractionModeTransition> listener
+            in listeners.GetInvocationList())
+        {
+            try
+            {
+                listener(transition);
+            }
+            catch (Exception error)
+            {
+                (failures ??= []).Add(error);
+            }
+        }
+
+        if (failures is not null)
+        {
+            throw new AggregateException(
+                "One or more interaction-mode reset observers failed.",
+                failures);
+        }
+    }
+
+    private bool Set(InteractionMode mode)
+    {
+        if (Current == mode)
+            return false;
+        InteractionMode previous = Current;
+        Current = mode;
+        Changed?.Invoke(new InteractionModeTransition(previous, mode));
+        return true;
+    }
+}