feat: read N/M/ratio/radix/exponent number literals; clean suite measurement

Reader gaps caused the clojure-test-suite worker to crash whole deftests on
literals it could not parse (0N, 1.5M, 2r1010, 1/2), losing every assertion in
the file. read-number now handles:
- N (bigint) / M (bigdec) suffixes -> plain number (Jolt has no bignum/bigdec)
- ratios a/b -> double quotient
- radix integers NrDDD (2r1010, 16rFF, 36rZ) parsed by base
- exponents (1e3, 1.5e-2) and 0X hex

Also fixed suite measurement: when-var-exists now skips silently (its SKIP
print to stdout was corrupting the worker's count line, dropping whole files),
and the worker emits counts on an @@COUNTS sentinel line (robust against test
bodies that print, e.g. with-out-str). Runner parses the sentinel; deftest
crashes now report the underlying message.

Impact: clojure-test-suite 210->231 files run, pass 1955->3535, clean files
24->39. Baseline raised to 3450/38.

spec: numbers/literal-syntax (13 cases). jpm test green.

test/support/clojure_test.clj

@@ -79,7 +79,7 @@
 ;; Run every deftest registered since the last reset, isolating crashes.
 (defn run-registered []
   (doseq [t @registry]
-    (try (t) (catch :default e (clojure.test/err! "deftest crashed"))))
+    (try (t) (catch :default e (clojure.test/err! (str "deftest crashed: " (clojure.core/ex-message e))))))
   nil)
 
 ;; clojure.test entry points the suite may call — no-ops; the Janet runner
@@ -96,10 +96,12 @@
 ;; Gate a test on whether its target var exists in this dialect. Jolt only
 ;; implements a subset of clojure.core, so unimplemented fns get skipped
 ;; cleanly rather than erroring.
+;; Skips silently (no stdout) so the worker's stdout stays just the count line;
+;; an unimplemented var simply contributes no assertions.
 (defmacro when-var-exists [var-sym & body]
   (if (resolve var-sym)
     `(do ~@body)
-    `(println "SKIP -" '~var-sym)))
+    nil))
 
 ;; `(thrown? body)` — true iff evaluating body throws. The suite always uses
 ;; the single-arg (no exception-class) form via this portability helper.