Process-local monotonic counter backing UXID's monotonic: mode.

All state lives in the process dictionary — no GenServer, no ETS, no shared state — so callers stay async: true safe. Each BEAM process maintains its own independent sequence per {prefix, rand_size}.

The random field is treated as one big-endian unsigned integer. The first ID in a given millisecond seeds it with full CSPRNG bytes; each subsequent ID in the same millisecond advances it by a random positive step drawn from the CSPRNG. Because the step is always >= 1 the sequence is strictly increasing, so IDs stay unique and — since the encoding is big-endian and the Crockford alphabet is ASCII-monotonic — integer order equals encoded lexical order, preserving K-sortability within the millisecond.

The step is uniform over [1, 2^(bits/2)] where bits is the field width, so the next value is spread across a window of size ~2^(bits/2) instead of being exactly +1. This is a mitigation, not cryptographic unpredictability: it removes trivial …0004 → …0005 enumeration and raises single-shot guess cost from certainty to ~1/2^(bits/2), but consecutive values still lie in a bounded window ahead. Low-entropy sizes (:small = 16 bits) remain low-entropy — don't use small/medium monotonic IDs as externally-enumerable, security-sensitive identifiers.