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/Util/Vec2.cs

@@ -0,0 +1,43 @@
+namespace Theriapolis.Core.Util;
+
+/// <summary>
+/// Lightweight 2D float vector for world-pixel-space polyline coordinates.
+/// Intentionally avoids System.Numerics to keep Core dependency-free.
+/// </summary>
+public readonly struct Vec2
+{
+    public readonly float X;
+    public readonly float Y;
+
+    public Vec2(float x, float y) { X = x; Y = y; }
+
+    public static Vec2 operator +(Vec2 a, Vec2 b) => new(a.X + b.X, a.Y + b.Y);
+    public static Vec2 operator -(Vec2 a, Vec2 b) => new(a.X - b.X, a.Y - b.Y);
+    public static Vec2 operator *(Vec2 a, float s) => new(a.X * s, a.Y * s);
+    public static Vec2 operator *(float s, Vec2 a) => new(a.X * s, a.Y * s);
+    public static Vec2 operator /(Vec2 a, float s) => new(a.X / s, a.Y / s);
+
+    public float LengthSquared => X * X + Y * Y;
+    public float Length => MathF.Sqrt(LengthSquared);
+
+    public Vec2 Normalized
+    {
+        get
+        {
+            float len = Length;
+            return len < 1e-6f ? new Vec2(0, 0) : this * (1f / len);
+        }
+    }
+
+    /// <summary>90° CCW rotation — perpendicular vector.</summary>
+    public Vec2 Perp => new(-Y, X);
+
+    public static float Dot(Vec2 a, Vec2 b) => a.X * b.X + a.Y * b.Y;
+    public static float DistSq(Vec2 a, Vec2 b) => (a - b).LengthSquared;
+    public static float Dist(Vec2 a, Vec2 b) => (a - b).Length;
+
+    /// <summary>Linear interpolation between a and b at parameter t.</summary>
+    public static Vec2 Lerp(Vec2 a, Vec2 b, float t) => a + (b - a) * t;
+
+    public override string ToString() => $"({X:F1}, {Y:F1})";
+}