test: adapt jank's form/reader tests into spec suites; fix case no-match

Vendoring jank's behavior (not the project): we base our own copies on the
jank test corpus to close coverage gaps, but maintain them ourselves since
jank may diverge. Two new spec batteries (jank-isms translated to Jolt:
letfn* -> letfn, jank catch types -> :default; platform-specific bigdec/
biginteger/ratio/uuid/##Inf/unicode cases omitted):

- test/spec/forms-spec.janet (52): case, fn (arity/variadic/closure/recur/
  named), let, letfn, loop, try, if/do/def/call — incl :throws regression
  cases (no-match, bad params, nil call).
- test/spec/reader-forms-spec.janet (22): #() (% %N %&), #' var-quote,
  ^metadata, syntax-quote (gensym/unquote/splice).

Fix surfaced by adaptation: case with no matching clause and no default now
throws 'No matching clause' (Clojure semantics) instead of returning nil
(00-syntax). Gate green incl full jpm build+test.

Other gaps the adaptation surfaced are filed (tests adjusted to jolt's
current behavior + a comment, not silently dropped):
  jolt-vdo case duplicate test constants not rejected
  jolt-w2v loop bindings not sequential (later init can't see earlier)
  jolt-6x1 #() %& miscomputes arity with a higher positional (%2 …)
  jolt-xl0 ^meta not attached to collection literals ({}/[]/#{})
  jolt-265 syntax-quote doesn't fully-qualify core syms to clojure.core/
  jolt-edb syntax-quote ~/~@ not processed inside set literals

jolt-core/clojure/core/00-syntax.clj

@@ -298,7 +298,8 @@
                     `(= ~g ~(mk-const c))))
         build (fn build [cls]
                 (if (empty? cls)
-                  nil
+                  ;; no clause matched and no default — Clojure throws here.
+                  `(throw (ex-info (str "No matching clause: " ~g) {}))
                   (if (empty? (rest cls))
                     (first cls)
                     `(if ~(mk-test (first cls)) ~(nth cls 1) ~(build (drop 2 cls))))))]