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.Core/World/Bridge.cs

@@ -0,0 +1,29 @@
+using Theriapolis.Core.Util;
+
+namespace Theriapolis.Core.World;
+
+/// <summary>
+/// A bridge where a road or rail polyline crosses a river.
+/// Endpoints are in world-pixel space and follow the actual road polyline,
+/// so the deck visually matches the road at the crossing regardless of meander.
+/// </summary>
+public readonly struct Bridge
+{
+    public readonly Vec2 Start;
+    public readonly Vec2 End;
+    public readonly int  RoadId;
+
+    public Bridge(Vec2 start, Vec2 end, int roadId)
+    {
+        Start  = start;
+        End    = end;
+        RoadId = roadId;
+    }
+
+    /// <summary>Center of the deck (midpoint of Start–End).</summary>
+    public float WorldPixelX => (Start.X + End.X) * 0.5f;
+    public float WorldPixelY => (Start.Y + End.Y) * 0.5f;
+
+    /// <summary>Road direction angle (radians) from Start to End.</summary>
+    public float RoadAngle => MathF.Atan2(End.Y - Start.Y, End.X - Start.X);
+}