Initial commit: Theriapolis baseline at port/godot branch point

Captures the pre-Godot-port state of the codebase. This is the rollback
anchor for the Godot port (M0 of theriapolis-rpg-implementation-plan-godot-port.md).
All Phase 0 through Phase 6.5 work is included; Phase 7 is in flight.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Theriapolis.Game/Screens/IScreen.cs

@@ -0,0 +1,26 @@
+using Microsoft.Xna.Framework;
+using Microsoft.Xna.Framework.Graphics;
+
+namespace Theriapolis.Game.Screens;
+
+/// <summary>
+/// A single game screen (title, world map, tactical, etc.).
+/// Managed by ScreenManager as a push/pop stack.
+/// </summary>
+public interface IScreen
+{
+    /// <summary>Called once when the screen is first pushed onto the stack.</summary>
+    void Initialize(Game1 game);
+
+    /// <summary>Called every frame while this screen is active (top of stack).</summary>
+    void Update(GameTime gameTime);
+
+    /// <summary>Called every frame to draw the screen.</summary>
+    void Draw(GameTime gameTime, SpriteBatch spriteBatch);
+
+    /// <summary>Called when a screen is popped or another is pushed on top.</summary>
+    void Deactivate();
+
+    /// <summary>Called when this screen comes back to the top of the stack.</summary>
+    void Reactivate();
+}