- Fix GCC -O2 warnings: calloc for cf (maybe-uninitialized),
suppress stringop-truncation/alloc-size-larger-than (false positives),
guard louvain against negative node_count
- Default compiler: cc on macOS, gcc on Linux/Windows (fixes gcc-14
not found on MSYS2)
- Docker test infra now runs BOTH test (ASan) and build (-O2 -Werror)