KMP string search

Both stages return all ascending start indices, including overlaps, comparing Text code points exactly. Empty pattern matches every boundary including the end; nonempty pattern in empty text yields no matches. Prototype O(nm); final KMP O(n+m) time/O(m) auxiliary space. No normalization or input mutation.

Public signatures:

Vec<Int> prototype(const Text& text,const Text& pattern);
Vec<Int> solve(const Text& text,const Text& pattern);

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.