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/CodexUI/Core/CodexWidget.cs

@@ -0,0 +1,47 @@
+using Microsoft.Xna.Framework;
+using Microsoft.Xna.Framework.Graphics;
+
+namespace Theriapolis.Game.CodexUI.Core;
+
+/// <summary>
+/// Base class for every CodexUI widget. Layout is two-pass:
+///   1. <see cref="Measure"/> — child reports a desired size given an
+///      available size envelope.
+///   2. <see cref="Arrange"/> — parent places the child by writing into
+///      <see cref="Bounds"/>; the child propagates to its own children.
+///
+/// Update + Draw run after layout. Hit-testing uses screen-space
+/// <see cref="Bounds"/> directly via <see cref="ContainsPoint"/>.
+/// </summary>
+public abstract class CodexWidget
+{
+    public Rectangle Bounds { get; protected set; }
+    public bool Visible { get; set; } = true;
+    public bool Enabled { get; set; } = true;
+    public CodexWidget? Parent { get; internal set; }
+
+    public Point DesiredSize { get; protected set; }
+
+    /// <summary>Child reports its preferred size; parent decides whether to honour it.</summary>
+    public Point Measure(Point available)
+    {
+        DesiredSize = MeasureCore(available);
+        return DesiredSize;
+    }
+
+    /// <summary>Parent commits a final rectangle; child propagates to grandchildren.</summary>
+    public void Arrange(Rectangle bounds)
+    {
+        Bounds = bounds;
+        ArrangeCore(bounds);
+    }
+
+    protected abstract Point MeasureCore(Point available);
+    protected abstract void  ArrangeCore(Rectangle bounds);
+
+    public virtual void Update(GameTime gt, CodexInput input) { }
+    public virtual void Draw(SpriteBatch sb, GameTime gt) { }
+
+    /// <summary>True if a screen-space point lies inside our bounds. Hover-/click-test helper.</summary>
+    public bool ContainsPoint(Point p) => Bounds.Contains(p);
+}