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.

jolt-core/jolt/backend_scheme.clj

@@ -636,11 +636,16 @@
         (if idx
           (order-args (fn [as] (str "(jrec-field-at " (first as) " " idx " " (emit fnode) ")")))
           (order-args (fn [as] (str "(jolt-get " (first as) " " (emit fnode) (defstr as) ")")))))
-      ;; (coll k [default]) -> (jolt-get coll k [default]) — coll (fnode) is the
-      ;; callee, evaluated before the key/default args.
+      ;; (coll k [default]) -> lookup — coll (fnode) is the callee, evaluated
+      ;; before the key/default args. A VECTOR literal invokes as nth (a bad
+      ;; index throws, IPersistentVector.invoke); maps/sets invoke as get.
       (= kind :coll)
       (ordered-call (cons fnode arg-nodes) (cons (emit fnode) args)
-                    (fn [[c & as]] (str "(jolt-get " c " " (str/join " " as) ")")))
+                    (fn [[c & as]]
+                      (str (if (and (= :vector (:op fnode)) (= 1 (count as)))
+                             "(jolt-nth "
+                             "(jolt-get ")
+                           c " " (str/join " " as) ")")))
       (and (stdlib-var? fnode) (not (deref prelude-mode?)))
       (throw (ex-info (str "emit: unsupported stdlib fn `" (:ns fnode) "/" (:name fnode)
                            "` (no core on Chez yet)") {}))