Fix 4 clojure.core bugs surfaced by JVM certification

The corpus certifier (test/conformance) flagged four cases where jolt's
hand-written :expected matched a real defect rather than Clojure. Fixed in the
jolt-core overlay, corrected the spec :expected, re-certified against JVM Clojure:

- ex-message: returns nil for a non-throwable (dropped the lenient string branch);
  still returns the message for ex-info. (jolt-l8e8)
- munge: preserves the argument's type — a symbol munges to a symbol, not a string.
  (jolt-hc35)
- print: (print nil) emits "nil", not "" (top-level nil guard; str yields "").
  (jolt-pqio)
- bounded-count: uses the counted? fast path (full count), else counts up to n via
  seq — was (min n (count coll)), wrong for counted colls. Added an uncounted-coll
  spec case. (jolt-2507)

Removed the 4 :bug entries from known-divergences.edn (now certified), regenerated
corpus + profile, re-minted the Chez bootstrap seed (clojure.core changed). Gates:
Janet 155/0, JVM certify clean, both Chez corpus gates 2534 (floors raised),
bootstrap 6/6, fixpoint intact.

test/conformance/known-divergences.edn

@@ -1,9 +1,7 @@
 {:doc
  "Known divergences of test/chez/corpus.edn :expected from JVM Clojure, classified. Most are deliberate jolt-specific or host-model differences (-> :features in conformance inc3). :bug entries are genuine and tracked. The certifier (certify.clj) gates on NEW (unlisted) divergences only. Keyed by [suite label].",
  :legend
- {:bug
-  "genuine jolt/corpus bug — tracked bead, fix in jolt-core + correct :expected",
-  :numeric-model
+ {:numeric-model
   "jolt is all-double: no Ratio/BigDecimal/float; (/ 1 2)=>0.5, 3.0 prints 3",
   :concurrency-model
   "Janet isolated-heap snapshot futures/agents/pmap; atoms snapshot, not shared",
@@ -18,23 +16,7 @@
   :impl-detail
   "representation detail: syntax-quote yields a list? (JVM yields a Cons)"},
  :entries
- [{:suite "clojure.core / leaf batch (complement fnil munge etc.)",
-   :label "munge symbol",
-   :category :bug,
-   :bead "jolt-hc35"}
-  {:suite "exceptions / ex-info",
-   :label "ex-message of string",
-   :category :bug,
-   :bead "jolt-l8e8"}
-  {:suite "io / print family (overlay)",
-   :label "print nil arg",
-   :category :bug,
-   :bead "jolt-pqio"}
-  {:suite "seq / overlay-migrated fns",
-   :label "bounded-count",
-   :category :bug,
-   :bead "jolt-2507"}
-  {:suite "clojure.core / futures — predicates",
+ [{:suite "clojure.core / futures — predicates",
    :label "cancel an in-flight future returns true",
    :category :concurrency-model,
    :flaky true}