fix: make test temp paths Windows-compatible

[TrueFurina]

Summary

Per #5: the tests hard-code POSIX paths (/tmp/...), so the suite cannot run on Windows even though the core library supports it.

Changes

• tests/test_core.cpp:
  • unique_temp_path() now resolves the temp directory from TEMP → TMP env vars, falling back to /tmp on POSIX (where those vars are unset)
  • added #include <cstdlib> for std::getenv
  • temp file names unchanged, so POSIX behaviour is identical

Verification

• POSIX: TEMP/TMP are normally unset → falls back to /tmp, same as before
• Windows: uses %TEMP%/%TMP% → tests no longer write to a non-existent /tmp
• Not compiled locally (no C++ toolchain here); change is minimal and stdlib-only