ocaml: character type — Char of int, #\a parser, all char primitives

- Char of int variant in sx_types.ml (Unicode codepoint)
- type_of → "char", inspect → #\a / #\space / #\newline notation
- #\ char literal reader in sx_parser.ml (named + single-char)
- make-char 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 Char values) and list->string (accepts Char values)
- fix get_val in sx_runtime.ml: add Integer n case for list indexing
- fix raw_serialize in sx_server.ml: Integer and Char variants
- 4493/4493 tests — +43 passing, zero regressions

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

hosts/ocaml/lib/sx_parser.ml

@@ -120,6 +120,27 @@ let rec read_value s : value =
   | '"' -> String (read_string s)
   | '\'' -> advance s; List [Symbol "quote"; read_value s]
   | '`' -> advance s; List [Symbol "quasiquote"; read_value s]
+  | '#' when s.pos + 1 < s.len && s.src.[s.pos + 1] = '\\' ->
+    (* Character literal: #\a, #\space, #\newline, etc. *)
+    advance s; advance s;
+    if at_end s then raise (Parse_error "Unexpected end of input after #\\");
+    let char_start = s.pos in
+    (* Read a name if starts with ident char, else single char *)
+    if is_ident_start s.src.[s.pos] then begin
+      while s.pos < s.len && is_ident_char s.src.[s.pos] do advance s done;
+      let name = String.sub s.src char_start (s.pos - char_start) in
+      let cp = match name with
+        | "space" -> 32 | "newline" -> 10 | "tab" -> 9
+        | "return" -> 13 | "nul" -> 0 | "null" -> 0
+        | "escape" -> 27 | "delete" -> 127 | "backspace" -> 8
+        | "altmode" -> 27 | "rubout" -> 127
+        | _ -> Char.code name.[0]  (* single letter like #\a *)
+      in Char cp
+    end else begin
+      let c = s.src.[s.pos] in
+      advance s;
+      Char (Char.code c)
+    end
   | '#' when s.pos + 1 < s.len && s.src.[s.pos + 1] = ';' ->
     (* Datum comment: #; discards next expression *)
     advance s; advance s;