[lenamonj]

On an instance opened with MMKV_READ_ONLY, set() that overwrites the only key of a single-process store writes into the read-only mapping and the process dies on the write fault. The append path refuses read-only inside ensureMemorySize(), but that overwrite takes the in-place override path, which never grows the file.

setDataForKey, which every set() overload reaches, now returns false on a read-only instance with the same warning removeValueForKey and removeValuesForKeys give.

The new testReadOnlyRejectsSet in POSIX/demo/UnitTest.cpp kills UnitTest on dev (SIGSEGV on Linux) and passes with the change; the four ctest targets pass.