Build directories are not ignored

`.gitignore` has no entry for a build directory. The README tells the reader to configure into `build/`:

```
cmake -S . -B build
```

so `build/` appears as untracked content in a normal working tree. It was 48 MB in one checkout here. Anyone running `git add -A` commits it.

Adding `build/` — or a broader pattern if out-of-tree names vary — closes it.