feat(battle-engine): seed copy roots + UnityEngine primitive shim

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.BattleEngine/Shim/UnityEngine/Primitives.cs

@@ -0,0 +1,35 @@
+// AUTHORED SHIM (not copied). Minimal no-op UnityEngine surface. Grows via the
+// Task 4 compile loop -- add only members the compiler actually demands.
+namespace UnityEngine
+{
+    public struct Vector2 { public float x, y; public Vector2(float x, float y){ this.x=x; this.y=y; } }
+    public struct Vector3
+    {
+        public float x, y, z;
+        public Vector3(float x, float y, float z){ this.x=x; this.y=y; this.z=z; }
+        public static Vector3 zero => new Vector3(0,0,0);
+        public static Vector3 operator +(Vector3 a, Vector3 b) => new Vector3(a.x+b.x, a.y+b.y, a.z+b.z);
+    }
+    public struct Quaternion
+    {
+        public float x, y, z, w;
+        public static Quaternion identity => new Quaternion();
+        public static Quaternion Euler(float x, float y, float z) => new Quaternion();
+    }
+    public struct Color { public float r, g, b, a; public Color(float r,float g,float b,float a){ this.r=r; this.g=g; this.b=b; this.a=a; } }
+    public static class Mathf
+    {
+        public const float PI = 3.14159265f;
+        public static float Floor(float f) => (float)System.Math.Floor(f);
+        public static int FloorToInt(float f) => (int)System.Math.Floor(f);
+        public static float Max(float a, float b) => System.Math.Max(a, b);
+        public static float Min(float a, float b) => System.Math.Min(a, b);
+        public static float Clamp(float v, float lo, float hi) => System.Math.Max(lo, System.Math.Min(hi, v));
+    }
+    public static class Debug
+    {
+        public static void Log(object m) { }
+        public static void LogWarning(object m) { }
+        public static void LogError(object m) { }
+    }
+}