Implement a B+-tree key-value store in C++17 in this directory. Files: btree.hpp (an in-memory B+ tree with configurable fanout, insert, find, erase, and an ordered range scan iterator), fuzz.cpp (a randomized differential test: 100k random insert/erase/find/scan ops mirrored against std::map, abort with a diagnostic on the first divergence), Makefile (clang++ -std=c++17 -fsanitize=address), README.md (node layout notes). Build with make, run the fuzz test, and fix failures until it survives the full run clean under ASan.