test: comprehensive spec — regex + sorted colls + random/predicate gaps

Filling the biggest untested clojure.core areas found in a coverage audit
(168 of 506 provided fns had no spec). New + expanded suites:

- regex-spec.janet (20): #"…" literals, regex?, re-find/re-matches/re-seq
  (match/no-match/groups), re-pattern, and clojure.string split/replace with
  regex (incl $1 backrefs). Whole area was previously unspecced.
- sorted-spec.janet (14): sorted-map/sorted-set construction + ordering, sorted?,
  subseq/rsubseq. Pins the working subset — get/conj/assoc/keys/vals on sorted
  colls and the by-comparator ctors are not yet first-class (jolt-ti9).
- predicates-spec +14: seqable?, integer?, reduced?/unreduced, not-empty.
- numbers-spec +5: rand/rand-int/rand-nth invariants.

Fix: sorted? was bound to core-sorted-map? so it returned false for sorted-sets;
now true for both sorted maps and sets (core-sorted?).

Filed: jolt-ti9 (sorted collections incomplete: get/conj/assoc/keys/vals don't
operate on the sorted wrapper; sorted-*-by ignore the comparator).

Gate green incl full jpm build + jpm test.

test/spec/numbers-spec.janet

@@ -126,3 +126,10 @@
   ["bit-clear"          "13"     "(bit-clear 15 1)"]
   ["bit-test true"      "true"   "(bit-test 4 2)"]
   ["bigint 64-bit"      "\"9000000000\"" "(str (bigint 9000000000))"])
+
+(defspec "numbers / random (invariants — non-deterministic)"
+  ["rand-int in range"  "true" "(let [r (rand-int 5)] (and (integer? r) (>= r 0) (< r 5)))"]
+  ["rand-int zero"      "0"    "(rand-int 1)"]
+  ["rand in [0,1)"      "true" "(let [r (rand)] (and (>= r 0) (< r 1)))"]
+  ["rand-nth member"    "true" "(contains? #{:a :b :c} (rand-nth [:a :b :c]))"]
+  ["rand-nth single"    ":x"   "(rand-nth [:x])"])