The Go 1.25 incompatibility found in
* Homebrew/homebrew-core#226636

then:
* reported upstream in cockroachdb/swiss#48
* fixed upstream cockroachdb/swiss#49

This panic causes an "unlock of unlocked mutex" panic on unwinding,
which obscures the original panic. Change to a `Fatalf`.

Fixes #5225

Add a build tag that lets developers choose the zstd library. The
current cgo implementation dramatically slows down builds and tests,
and it blocks non‑cgo projects from running tests with the race
detector unless they also switch the underlying library.