Contract fixes from the baseline audit; every residual suite failure traced

Auditing the remaining cts baseline for R7 exposed real contract gaps hiding
among the model residue — all fixed to reference behavior:

- stale no-ratio-era stubs: numerator/denominator now work over jolt's exact
  rationals (non-ratio is the Ratio cast failure); rational? includes decimals
- casts and pending: peek/pop demand an IPersistentStack (pop nil is nil),
  realized? demands an IPending (a plain list/range throws), transient demands
  an editable COLLECTION (non-colls throw; the RFC 0003 sorted/list/seq
  superset keeps the copy-on-write fallback), empty on a plain record throws
- nil and empties: (nth nil i) is nil, (nth nil i d) is d, a nil index is NPE,
  keys/vals of anything empty are nil, (conj nil) is nil
- lookups: contains? on a string is index-only (other keys IAE), get on an
  array is lenient (nth still throws), a VECTOR invocation has nth semantics
  (([1 2] 5) throws — call position and jolt-invoke both)
- into only transients editable collections; a PersistentQueue/sorted target
  folds through conj (RT's IEditableCollection split)
- numbers: number?/num accept BigDecimal, quot/rem throw on an Infinite/NaN
  quotient, even?/odd? demand integers
- ordering: keywords compare namespace-first with nil first (Symbol.compareTo)
- misc: run! honors reduced, eval self-evaluates non-form values, intern
  demands an existing namespace, counted? excludes strings, seqable? includes
  arrays, shuffle rejects maps, sort-by rejects a collection comparator,
  when-let demands one binding pair, case*/deftype*/letfn*/reify*/& are
  special symbols

Two mis-certified corpus rows fixed (they threw on the JVM too and hid in the
tolerated bucket): a raw \d string escape and duplicate literal map keys.

SPEC.md gains the baseline-traceability section: every one of the 146
remaining suite failures maps to a documented divergence (integer-box,
no-single-float, RFC 0003 transients, seq/chunking model, stm-refs,
parse-uuid strictness, vec-array adoption). cts baseline 5955 -> 6042 pass,
5 errors, 30 namespaces. 9 JVM-certified corpus rows.

host/chez/converters.ss

@@ -178,7 +178,11 @@
     ((jolt-nil? b) 1)
     ((and (number? a) (number? b)) (jolt-cmp3 a b))
     ((and (string? a) (string? b)) (jolt-strcmp a b))
-    ((and (keyword? a) (keyword? b)) (jolt-strcmp (jolt-kw->string a) (jolt-kw->string b)))
+    ;; keywords order like symbols: a nil namespace sorts before any namespace,
+    ;; then by namespace, then by name (Keyword.compareTo -> Symbol.compareTo)
+    ((and (keyword? a) (keyword? b))
+     (let ((r (jolt-strcmp (or (keyword-t-ns a) "") (or (keyword-t-ns b) ""))))
+       (if (= r 0) (jolt-strcmp (keyword-t-name a) (keyword-t-name b)) r)))
     ((and (jolt-symbol? a) (jolt-symbol? b))
      (let ((r (jolt-strcmp (jolt-sym-ns-string a) (jolt-sym-ns-string b))))
        (if (= r 0) (jolt-strcmp (symbol-t-name a) (symbol-t-name b)) r)))
@@ -269,4 +273,16 @@
         (jolt-cast-range-throw "float" x)
         d)))
 (def-var! "clojure.core" "float" jolt-float)
+;; numerator/denominator: jolt ratios are Chez exact rationals; a non-ratio is
+;; the JVM's Ratio cast failure.
+(define (jolt-ratio-part name f)
+  (lambda (x)
+    (if (and (number? x) (exact? x) (rational? x) (not (integer? x)))
+        (f x)
+        (jolt-throw (jolt-host-throwable
+                     "java.lang.ClassCastException"
+                     (string-append "class " (guard (e (#t "?")) (jolt-class-name x))
+                                    " cannot be cast to class clojure.lang.Ratio"))))))
+(def-var! "clojure.core" "numerator" (jolt-ratio-part "numerator" numerator))
+(def-var! "clojure.core" "denominator" (jolt-ratio-part "denominator" denominator))
 (def-var! "clojure.core" "compare" jolt-compare)