Portable UUIDv7 generator for Clojure, ClojureScript, Babashka, nbb, and scittle.

Implements RFC 9562 Section 6.2 Method 3 (monotonic random):
- 48-bit millisecond Unix timestamp
- 74-bit monotonically increasing random counter
- Sub-millisecond ordering guaranteed from a single generator
- No blocking, no spinning, no overflow in practice

Usage:
  (require '[com.github.franks42.uuidv7.core :refer [uuidv7]])
  (uuidv7)  ;=> #uuid "0195xxxx-xxxx-7xxx-xxxx-xxxxxxxxxxxx"

  ;; Extract embedded data:
  (extract-ts   u)  ;=> 1738934578991        (ms since epoch)
  (extract-inst u)  ;=> #inst "2025-02-07..." (as Date)
  (extract-key  u)  ;=> [ts a bh bl]          (sortable composite key)

The 74-bit counter space (~1.9 * 10^22 values per millisecond)
is effectively inexhaustible. On each new millisecond the counter
reseeds with fresh random bits. Within the same millisecond it
increments by a random amount (1 to 2^31), preserving both
monotonicity and unpredictability.