[Huang-404-Q]

InMemoryEnv::RenameFile(src, target) removes the target before copying the source entry, so when src == target it deletes the only map entry and then re-inserts nothing. The call returns OK, but the file is gone. POSIX rename(2) with identical paths is a successful no-op that preserves the file.

Repro (added as MemEnvTest.RenameFileToItself):

env->NewWritableFile("/dir/f", &w);
w->Append("abc");
delete w;
env->RenameFile("/dir/f", "/dir/f");  // OK
env->FileExists("/dir/f");            // false before the fix, true after
env->GetFileSize("/dir/f", &size);    // IOError before, 3 after

Fix: return OK before touching the map when src == target.

Verification (MinGW, CMake + Ninja):

cmake --build build --target leveldb_tests -j 8
./build/leveldb_tests.exe --gtest_filter='MemEnvTest.RenameFileToItself'

Without the fix the test fails (FileExists is false); with it, MemEnvTest.* passes 8/8. No existing open PR touches helpers/memenv.