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/predicates-spec.janet

@@ -113,3 +113,19 @@
   ["not= differs"       "true"   "(not= [1 2] [1 3])"]
   ["identical? same kw" "true"   "(identical? :a :a)"]
   ["compare strings"    "-1"     "(compare \"a\" \"b\")"])
+
+(defspec "predicates / seqable, reduced & emptiness"
+  ["seqable? vector"   "true"   "(seqable? [1])"]
+  ["seqable? map"      "true"   "(seqable? {:a 1})"]
+  ["seqable? string"   "true"   "(seqable? \"s\")"]
+  ["seqable? nil"      "true"   "(seqable? nil)"]
+  ["seqable? number"   "false"  "(seqable? 5)"]
+  ["integer? int"      "true"   "(integer? 5)"]
+  ["integer? fraction" "false"  "(integer? 5.5)"]
+  ["reduced? wrapped"  "true"   "(reduced? (reduced 1))"]
+  ["reduced? plain"    "false"  "(reduced? 1)"]
+  ["unreduced wrapped" "9"      "(unreduced (reduced 9))"]
+  ["unreduced plain"   "9"      "(unreduced 9)"]
+  ["not-empty full"    "[1]"    "(not-empty [1])"]
+  ["not-empty empty"   "nil"    "(not-empty [])"]
+  ["not-empty string"  "nil"    "(not-empty \"\")"])