Factor TempDir's name generation into a reusable `randomBasename` (16 random bytes, url-safe base64) and add `randomTmpPath` on top, which composes `allocTmpDir` + `randomBasename` into a single allocated path in the form `{TMPDIR}/{prefix}{random}` (mktemp(1)-ish). This is convenient for callers who want a unique path under TMPDIR (for a temporary file, socket, etc.) without having to think about basename buffer sizing or path joining. Also, use `std.base64.url_safe_no_pad.Encoder` instead of the custom base64 alphabet, which is exactly equivalent.