Unify CEK callable dispatch, add named-let transpiler, full stdlib

Three changes that together enable the full 46-function stdlib migration:

1. CEK callable unification (spec/evaluator.sx):
   cek-call now routes both native callables and SX lambdas through
   continue-with-call, so replacing a native function with an SX lambda
   doesn't change shift/reset behavior.

2. Named-let transpiler support (hosts/javascript/transpiler.sx):
   (let loop ((i 0)) body...) now transpiles to a named IIFE:
   (function loop(i) { body })(0)
   This was the cause of the 3 test regressions (produced [object Object]).

3. Full stdlib via runtime eval (hosts/javascript/bootstrap.py):
   stdlib.sx is eval'd at runtime (not transpiled) so its defines go
   into PRIMITIVES without shadowing module-scope variables that the
   transpiled evaluator uses directly.

stdlib.sx now contains all 46 library functions:
  Logic: not
  Comparison: != <= >= eq? eqv? equal?
  Predicates: boolean? number? string? list? dict? continuation?
    zero? odd? even? empty?
  Arithmetic: inc dec abs ceil round min max clamp
  Collections: first last rest nth cons append reverse flatten
    range chunk-every zip-pairs
  Dict: vals has-key? assoc dissoc into
  Strings: upcase downcase string-length substring string-contains?
    starts-with? ends-with? split join replace contains?
  Text: pluralize escape parse-datetime assert

All hosts: JS 957+1080, Python 744, OCaml 952 — zero regressions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

spec/evaluator.sx

@@ -1540,13 +1540,18 @@
       (kont-push (make-deref-frame env) kont))))
 
 ;; cek-call — call a function via CEK (replaces invoke)
+;; cek-call — unified function dispatch
+;; Both lambdas and native callables go through continue-with-call
+;; so they interact identically with the continuation stack.
+;; This is critical: replacing a native callable with an SX lambda
+;; (e.g. stdlib.sx) must not change shift/reset behavior.
 (define cek-call
   (fn (f args)
     (let ((a (if (nil? args) (list) args)))
       (cond
         (nil? f) nil
-        (lambda? f) (cek-run (continue-with-call f a (dict) a (list)))
-        (callable? f) (apply f a)
+        (or (lambda? f) (callable? f))
+          (cek-run (continue-with-call f a (dict) a (list)))
         :else nil))))
 
 ;; reactive-shift-deref: the heart of deref-as-shift