Function splitSecret

Split secret into shareCount shares, any threshold of which recover it. The secret must be 16–32 bytes and even-length.

With threshold 1 every share is a copy of the secret and no randomness is drawn. Otherwise threshold - 2 shares are drawn whole from rng, then secret.length - 4 bytes for the digest share's tail; the remaining shares are interpolated. That draw order is wire. Draws go through rand's fillRandomBytes, so rand's generator contract applies: a generator's own error propagates unwrapped, and one without a callable fillBytes fails at the first draw with rand's RandError InvalidGenerator.