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.

test/integration/clojure-test-suite-test.janet

@@ -43,9 +43,9 @@
 # Raised 4004 -> 4034 / clean 66 -> 67 porting partition-all + repeatedly to the
 # overlay, which required fixing two leniencies (a char is not callable; take
 # validates its count) — correct beyond those fns, so the suite rose broadly.
-(def baseline-pass 4034)
+(def baseline-pass 4074)
 # A file is "clean" when it ran with zero failures AND zero errors.
-(def baseline-clean-files 67)
+(def baseline-clean-files 71)
 # Per-file wall-clock budget (seconds). Normal files finish in well under 1s, so
 # this normally only fires on genuinely-infinite-sequence hangs. It's an env var
 # (JOLT_SUITE_TIMEOUT) so CI — whose runners are slower than a dev machine — can