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/transients.ss

@@ -44,7 +44,16 @@
      (let ((ht (make-hashtable key-hash jolt=2)))
        (pset-fold coll (lambda (e acc) (hashtable-set! ht e #t) acc) 0)
        (make-jolt-transient 'set ht 0 #t #f)))
-    (else (make-jolt-transient 'cow coll 0 #t #f))))
+    ;; RFC 0003: any COLLECTION transients (the sorted/list/seq superset rides
+    ;; the copy-on-write fallback); a non-collection is the JVM's cast failure.
+    ((or (cseq? coll) (empty-list-t? coll) (jolt-lazyseq? coll)
+         (htable? coll) (jrec? coll))
+     (make-jolt-transient 'cow coll 0 #t #f))
+    (else
+     (jolt-throw (jolt-host-throwable
+                  "java.lang.ClassCastException"
+                  (string-append "class " (guard (e (#t "?")) (jolt-class-name coll))
+                                 " cannot be cast to class clojure.lang.IEditableCollection"))))))
 
 ;; map put/delete that maintain the reverse insertion-order list in `ord`.
 (define (tmap-put! t k v)