core: proper uuid support — fix random-uuid, add parse-uuid, real uuid values

uuid support was broken end to end (jolt-6s2): random-uuid built malformed
strings ((string/format "%x") with no zero-padding, so hex groups came out
short, and no variant bits), uuid? was hardcoded false, the #uuid data reader
was identity (bare string), and parse-uuid didn't exist. Nothing tested it.

A UUID is now an immutable tagged struct {:jolt/type :jolt/uuid :str <lower>}
(make-uuid, types.janet) — struct value equality gives case-insensitive = and
map-key/set hashing for free, and the evaluator treats it as self-evaluating
(like chars). random-uuid emits a correct RFC 4122 v4 (zero-padded 8-4-4-4-12,
version nibble 4, variant 8-b). parse-uuid validates the canonical shape,
returns nil on a bad string, throws on a non-string (Clojure 1.11). uuid? is
an overlay tag predicate. str renders the bare string; pr-str renders
#uuid "..." and round-trips.

Tests: uuid-spec (30 cases: format, parse edge cases from the suite, value
semantics, reader literal), 6 conformance cases x3 modes. The suite's
parse_uuid/random_uuid/uuid_qmark files now contribute: 4049 -> 4074 pass,
71 clean files; baselines raised.

jolt-core/clojure/core/20-coll.clj

@@ -286,6 +286,7 @@
 (defn reader-conditional? [x] (= (get x :jolt/type) :jolt/reader-conditional))
 (defn tagged-literal? [x]     (= (get x :jolt/type) :jolt/tagged-literal))
 (defn record? [x]             (some? (get x :jolt/deftype)))
+(defn uuid? [x]               (= (get x :jolt/type) :jolt/uuid))
 ;; Jolt has no chunked seqs (Phase 5 territory), so this is always false.
 (defn chunked-seq? [x] false)