/**
 * Per-user private temp directories.
 *
 * Shared paths like /tmp/nebula-kernels break on multi-user machines twice
 * over: the first user's mkdir (with a restrictive umask) locks everyone else
 * out, and anything world-readable leaks — kernel connection files carry the
 * HMAC key, which is remote code execution as the kernel's owner for anyone
 * who can read it.
 *
 * Layout: <tmpdir>/nebula-<username>/<segments...>, mode 0700 throughout.
 * /tmp is sticky, so another user CAN pre-create our directory name — we
 * verify ownership after mkdir and refuse to use a directory we don't own.
 */
export declare function privateTmpDir(...segments: string[]): string;