Background

The Windows + macOS CI matrix expansion (#634) skips three test classes
in `packages/memtomem/tests/test_runtime_paths.py` on Windows:

• `TestRuntimeDir`
• `TestEnsureRuntimeDir`
• `TestServerPidPath`

These classes assume POSIX semantics that don't translate cleanly to
NTFS:

• The shared `_make_safe_xdg` fixture does `os.chmod(xdg, 0o700)` to
  set up the XDG base. NTFS reports synthesized POSIX mode bits
  (typically `0o666` or `0o777`) that don't match `0o700`, so
  `_is_safe_dir` (which checks `S_IMODE(st.st_mode) & 0o077`) would
  reject the directory and the XDG branch silently falls through to the
  tempdir form — breaking the tests that assert the XDG path is used.
• `TestEnsureRuntimeDir.test_creates_directory_with_owner_only_mode`
  asserts `S_IMODE(d.stat().st_mode) == 0o700`, which is POSIX-only.
• `TestEnsureRuntimeDir.test_explicit_chmod_survives_wild_umask` uses
  `os.umask(0o177)`, which Windows ignores.
• `TestEnsureRuntimeDir.test_refuses_existing_loose_mode` depends on
  the `0o755` vs `0o700` distinction.

The runtime code itself
(`packages/memtomem/src/memtomem/_runtime_paths.py`) already has a
Windows-aware path through `tempfile.gettempdir()` + `uid=0` (the
`os.geteuid()` calls are `hasattr`-guarded), but its behavior on
NTFS is unverified today — that's what this issue tracks.

Scope

1. Verify `runtime_dir()` and `ensure_runtime_dir()` behave correctly
   on Windows: pid file lands at `%LOCALAPPDATA%\Temp\memtomem-0`
   (or whatever `tempfile.gettempdir()` returns), `ensure_runtime_dir()`
   doesn't raise on a fresh runner, and validation doesn't no-op away
   the security guarantees we care about.
2. Decide the NTFS security contract. Options:
   • Relax `_is_safe_dir` on Windows (accept the kernel default).
   • Use `os.access(path, os.W_OK)` + owner-SID checks via stdlib
     `os.stat()` + `stat.FILE_ATTRIBUTE_*` (or `pywin32` if needed).
   • Document Windows as best-effort and skip mode-bit validation.
3. Either rewrite the three skipped classes to be Windows-aware, or
   add a separate `TestRuntimeDir{Windows,Posix}` split with native
   assertions per platform.

Out of scope: changing the runtime code's POSIX contract. The pid-file
location and ownership semantics on Windows are an open design
question that this issue should decide before relaxing anything.

References

• ci: add Windows runner to CI matrix #634 — Windows + macOS CI matrix expansion
• `packages/memtomem/src/memtomem/_runtime_paths.py:63-145` — runtime
  resolver + `ensure_runtime_dir`
• `packages/memtomem/tests/test_runtime_paths.py` — three test classes
  currently skipped on Windows

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