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>

spec/primitives.sx

@@ -1,36 +1,38 @@
 ;; ==========================================================================
-;; primitives.sx — Specification of all SX built-in pure functions
+;; primitives.sx — Irreducible primitive set
 ;;
-;; Each entry declares: name, parameter signature, and semantics.
-;; Bootstrap compilers implement these natively per target.
+;; These are the functions that CANNOT be written in SX because they
+;; require host-native capabilities: native arithmetic, type inspection,
+;; host string library, host math, host I/O, host data structures.
 ;;
-;; This file is a SPECIFICATION, not executable code. The define-primitive
-;; form is a declarative macro that bootstrap compilers consume to generate
-;; native primitive registrations.
+;; Everything else lives in spec/stdlib.sx as library functions.
+;;
+;; The primitive set is the out-of-band floor. The fewer primitives,
+;; the tighter the strange loop and the more of the system is auditable,
+;; verifiable, portable SX.
 ;;
 ;; Format:
 ;;   (define-primitive "name"
 ;;     :params (param1 param2 &rest rest)
 ;;     :returns "type"
-;;     :doc "description"
-;;     :body (reference-implementation ...))
+;;     :doc "description")
 ;;
-;; Typed params use (name :as type) syntax:
-;;   (define-primitive "+"
-;;     :params (&rest (args :as number))
-;;     :returns "number"
-;;     :doc "Sum all arguments.")
+;; Typed params use (name :as type) syntax.
+;; Modules: (define-module :name) scopes subsequent entries.
 ;;
-;; Untyped params default to `any`. Typed params enable the gradual
-;; type checker (types.sx) to catch mistyped primitive calls.
-;;
-;; The :body is optional — when provided, it gives a reference
-;; implementation in SX that bootstrap compilers MAY use for testing
-;; or as a fallback. Most targets will implement natively for performance.
-;;
-;; Modules: (define-module :name) scopes subsequent define-primitive
-;; entries until the next define-module. Bootstrappers use this to
-;; selectively include primitive groups.
+;; Functions moved to stdlib.sx (no longer primitives):
+;;   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
+;;   Logic:       not
+;;   Text:        pluralize escape assert parse-datetime
 ;; ==========================================================================