spec: character type (char? char->integer #\a literals + predicates)

- Add SxChar tagged object {_char, codepoint} to JS platform
- char? char->integer integer->char char-upcase char-downcase
- char=? char<? char>? char<=? char>=? comparators
- char-ci=? char-ci<? char-ci>? char-ci<=? char-ci>=? case-insensitive
- char-alphabetic? char-numeric? char-whitespace? char-upper-case? char-lower-case?
- string->list (returns chars) and list->string (accepts chars)
- #\a #\space #\newline reader syntax in spec/parser.sx
- integer->char alias in spec/evaluator.sx
- js-char-renames dict in transpiler.sx for ->-containing names
- 43 tests in spec/tests/test-chars.sx, all passing

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

spec/primitives.sx

@@ -492,6 +492,12 @@
   :returns "string"
   :doc "Convert Unicode code point to single-character string.")
 
+(define-primitive
+  "char-code"
+  :params ((s :as string))
+  :returns "number"
+  :doc "Unicode codepoint of the first character of string s.")
+
 (define-primitive
   "substring"
   :params ((s :as string) (start :as number) (end :as number))
@@ -546,15 +552,15 @@
   :returns "boolean"
   :doc "True if string s starts with prefix.")
 
+;; --------------------------------------------------------------------------
+;; Core — Dict operations
+;; --------------------------------------------------------------------------
 (define-primitive
   "ends-with?"
   :params ((s :as string) (suffix :as string))
   :returns "boolean"
   :doc "True if string s ends with suffix.")
 
-;; --------------------------------------------------------------------------
-;; Core — Dict operations
-;; --------------------------------------------------------------------------
 (define-module :core.collections)
 
 (define-primitive
@@ -599,15 +605,15 @@
   :returns "any"
   :doc "Last element, or nil if empty.")
 
+;; --------------------------------------------------------------------------
+;; Stdlib — Format
+;; --------------------------------------------------------------------------
 (define-primitive
   "rest"
   :params ((coll :as list))
   :returns "list"
   :doc "All elements except the first.")
 
-;; --------------------------------------------------------------------------
-;; Stdlib — Format
-;; --------------------------------------------------------------------------
 (define-primitive
   "nth"
   :params ((coll :as list) (n :as number))
@@ -632,15 +638,15 @@
   :returns "list"
   :doc "Mutate coll by appending x in-place. Returns coll.")
 
+;; --------------------------------------------------------------------------
+;; Stdlib — Text
+;; --------------------------------------------------------------------------
 (define-primitive
   "reverse"
   :params ((coll :as list))
   :returns "list"
   :doc "Return coll in reverse order.")
 
-;; --------------------------------------------------------------------------
-;; Stdlib — Text
-;; --------------------------------------------------------------------------
 (define-primitive
   "flatten"
   :params ((coll :as list))
@@ -659,29 +665,29 @@
   :returns "list"
   :doc "Consecutive pairs: (1 2 3 4) → ((1 2) (2 3) (3 4)).")
 
-(define-module :core.dict)
-
 ;; --------------------------------------------------------------------------
 ;; Stdlib — Style
 ;; --------------------------------------------------------------------------
 ;; --------------------------------------------------------------------------
 ;; Stdlib — Debug
 ;; --------------------------------------------------------------------------
+(define-module :core.dict)
+
 (define-primitive
   "keys"
   :params ((d :as dict))
   :returns "list"
   :doc "List of dict keys.")
 
+;; --------------------------------------------------------------------------
+;; Type introspection — platform primitives
+;; --------------------------------------------------------------------------
 (define-primitive
   "vals"
   :params ((d :as dict))
   :returns "list"
   :doc "List of dict values.")
 
-;; --------------------------------------------------------------------------
-;; Type introspection — platform primitives
-;; --------------------------------------------------------------------------
 (define-primitive
   "merge"
   :params (&rest (dicts :as dict))