`allocTmpDir` previously read `%TMP%` via `getenvW` and returned `null`
if the variable wasn't set, requiring each caller to to deal with the
nullable. Unfortunately, there isn't a platform-neutral default value
that makes sense for those cases (i.e. `/tmp` is POSIX-y).
We now use `GetTempPathW` on Windows, which is the official way to get
this directory: `TMP` → `TEMP` → `USERPROFILE` → `GetWindowsDirectoryW`.
With a real system call behind it, the function no longer needs to be
nullable: the only remaining failure modes are OOM (propagated) and the
syscall itself failing or returning data we can't decode. In those later
cases, we use `C:\Windows\Temp` as a fallback, similar to how we use
`/tmp` in the POSIX case.
The Windows path always allocates so it still must be paired with
`freeTmpDir`, which matches the existing contract.
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