Add spec/stdlib.sx: 46 primitives become library functions

The irreducible primitive set drops from 79 to 33. Everything that can
be expressed in SX is now a library function in stdlib.sx, loaded after
evaluator.sx and before render.sx.

Moved to stdlib.sx (pure SX, no host dependency):
- Logic: not
- Comparison: != <= >= eq? eqv? equal?
- Predicates: nil? boolean? number? string? list? dict? continuation?
  empty? odd? even? zero? contains?
- Arithmetic: inc dec abs ceil round min max clamp
- Collections: first last rest nth cons append reverse flatten range
  chunk-every zip-pairs vals has-key? merge assoc dissoc into
- Strings: upcase downcase string-length substring string-contains?
  starts-with? ends-with? split join replace
- Text: pluralize escape assert parse-datetime

Remaining irreducible primitives (33):
  + - * / mod floor pow sqrt = < > type-of symbol-name keyword-name
  str slice index-of upper lower trim char-from-code list dict concat
  get len keys dict-set! append! random-int json-encode format-date
  parse-int format-decimal strip-tags sx-parse error apply

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

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

hosts/python/platform.py

@@ -1640,6 +1640,7 @@ SPEC_MODULES = {
     "engine": ("engine.sx", "engine (fetch/swap/trigger pure logic)"),
     "signals": ("signals.sx", "signals (reactive signal runtime)"),
     "page-helpers": ("page-helpers.sx", "page-helpers (pure data transformation helpers)"),
+    "stdlib": ("stdlib.sx", "stdlib (library functions from former primitives)"),
     "types": ("types.sx", "types (gradual type system)"),
     "freeze": ("freeze.sx", "freeze (serializable state boundaries)"),
     "content": ("content.sx", "content (content-addressed computation)"),
@@ -1647,9 +1648,10 @@ SPEC_MODULES = {
 # Note: frames and cek are now part of evaluator.sx (always loaded as core)
 
 # Explicit ordering for spec modules with dependencies.
+# stdlib must come first — other modules use its functions.
 # freeze depends on signals; content depends on freeze.
 SPEC_MODULE_ORDER = [
-    "deps", "engine", "page-helpers", "router", "signals", "types", "freeze", "content",
+    "stdlib", "deps", "engine", "page-helpers", "router", "signals", "types", "freeze", "content",
 ]
 
 EXTENSION_NAMES = {"continuations"}