test+docs: spec coverage for migratus-enablement fixes; list migratus

Adds spec/conformance coverage for everything landed enabling migratus:
- conformance corpus (runs interpret/compile/self-host): def 3-arg docstring,
  def/defn ^{:map} name meta, defmacro arity-clause + docstring, defmulti
  docstring, assoc-nil/assoc-in real maps, try multi-body + finally-on-success,
  current-ns restore after a caught throw, cross-ns methods visibility.
- spec suites: host-interop (exception ctors, Character/Thread/Long, Timestamp/
  SimpleDateFormat, java.io.File model + File-aware file-seq, clojure.tools.logging),
  regex (Pattern statics + MULTILINE + quote, String .matches/.replaceAll/
  .replaceFirst), maps (assoc on nil), multimethods (defmulti docstring +
  value-based methods/get-method), macros (defmacro arity-clause + name meta).

Rewrote clojure.tools.logging/spy as a single variadic arity (jolt defmacro
takes only the first clause of a multi-arity macro — jolt-q8l).

docs/libraries.md: add migratus and the next.jdbc compatibility layer, with the
janet-lang/sqlite3 int64 caveat for 14-digit timestamp ids.

Full gate green; conformance 350/350 x3 modes.

test/spec/maps-spec.janet

@@ -177,3 +177,13 @@
   ["empty? lazy nil elem" "false"     "(empty? (cons nil nil))"]
   ["empty? sorted"   "[true false]"   "[(empty? (sorted-map)) (empty? (sorted-set 1))]"]
   ["empty? number throws" :throws     "(empty? 5)"])
+
+# (assoc nil k v) yields a real immutable map, not a raw host table, so
+# assoc-in into absent keys nests countable/seqable maps (jolt-w4s).
+(defspec "map / assoc on nil"
+  ["assoc nil is a map"        "{:a 1}" "(assoc nil :a 1)"]
+  ["count of assoc nil"        "1"      "(count (assoc nil :a 1))"]
+  ["assoc-in nested countable" "1"      "(count (:a (assoc-in {} [:a :b] 1)))"]
+  ["assoc-in deep get"         "9"      "(get-in (assoc-in {} [:a :b :c] 9) [:a :b :c])"]
+  ["seq over assoc-nil map"    ":a"     "(ffirst (seq (assoc nil :a 1)))"]
+  ["keys of assoc-nil map"     "[:a]"   "(vec (keys (assoc nil :a 1)))"])