3.15.0

fix: smoke-test prebuilds instead of running upstream suite

Upstream's suite can't run under nodejs-mobile. It requires fd-lock, and
fs-native-extensions ships no usable nodejs-mobile prebuild on npm: the
android .node it does ship links only libc/libdl/libm, so napi_* never
resolves and dlopen fails. require-addon reports that as "Cannot find addon"
with the real dlopen error buried in the error's `cause`, which the module
runner doesn't print — so it reads as a missing file. The suite also calls
os.tmpdir() ('/tmp', absent on mobile) and opens fixtures via repo-relative
paths, while the runtime starts with cwd='/'.

Run a functional smoke test instead: open, write, read back, missing-key,
close, and reopen to prove data hit disk. That covers what a prebuild has to
guarantee without dragging in a second native addon.

Verified on an android-arm64 emulator against the 3.17.2 prebuild: 6/6, exit
0. The same run confirmed rocksdb-native's own addon dlopens cleanly.