Monotonic deques

Return one maximum for every full contiguous window. Negative loads and duplicates are valid. width<=0 throws invalid_argument even for empty input. Empty input or width>size returns empty. Prototype rescans each window; final monotonic deque O(n) time/O(width) auxiliary space. Preserve loads.

Public signatures:

Vec<Int> prototype(const Vec<Int>& loads,const Int& width);
Vec<Int> solve(const Vec<Int>& loads,const Int& width);

Compile and run from this directory:

c++ -std=c++20 -O2 -Wall -Wextra -pedantic -pthread main.cpp -o practice
./practice --stage prototype
./practice --stage final

Edit exercise.hpp. Both stages start unfinished and must report failures until implemented. Keep main.cpp as the executable contract. The runner returns nonzero for a failed check, an unexpected exception, an invalid stage, or an empty suite. C++20 and only the standard library are required.