Scaffold day 1

CMakeLists.txt

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+cmake_minimum_required(VERSION 3.10.0)
+project(aoc25 VERSION 0.1.0 LANGUAGES C CXX)
+
+# Per-day subprojects (example: day1)
+add_subdirectory(day1)
+

day1/CMakeLists.txt

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+add_executable(day1 main.cpp)
+set_target_properties(day1 PROPERTIES CXX_STANDARD 17 CXX_STANDARD_REQUIRED ON)

day1/main.cpp

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+#include <iostream>
+
+int main(int, char**){
+    std::cout << "Hello, from aoc25 day1!\n";
+}