Add AoC 2025 project structure and first solutions

AdvenOfCode.Contracts/AdvenOfCode.Contracts.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net9.0</TargetFramework>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <Nullable>enable</Nullable>
+    </PropertyGroup>
+
+</Project>

AdvenOfCode.Contracts/IPuzzleSolver.cs

@@ -0,0 +1,7 @@
+namespace AdvenOfCode.Contracts;
+
+public interface IPuzzleSolver<T>
+{
+    public T SolvePart1(string puzzleInputPath);
+    public T SolvePart2(string puzzleInputPath);
+}