spec: read/write/display — S-expression reader/writer on ports

Adds read, write, display, newline, write-to-string, display-to-string
and current-*-port primitives to both JS and OCaml hosts.

JS: sxReadNormalize (#t/#f→true/false), sxReadConvert (()→nil),
    sxEq array comparison, sxWriteVal symbol/keyword name fix,
    readerMacroGet/readerMacroSet registry in parser platform.
OCaml: sx_write_val/sx_display_val helpers, read/write/display/newline
    primitives on port types; parser extended for #t/#f and N/D rationals.
42 new tests (test-read-write.sx), all passing on JS and OCaml.

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

hosts/ocaml/lib/sx_primitives.ml

@@ -127,6 +127,46 @@ let rat_div (an, ad) (bn, bd) =
   if bn = 0 then raise (Eval_error "rational: division by zero");
   make_rat (an * bd) (ad * bn)
 
+(* write/display serializers *)
+let rec sx_write_val = function
+  | Nil -> "()"
+  | Eof -> "#!eof"
+  | Bool true -> "#t"
+  | Bool false -> "#f"
+  | Integer n -> string_of_int n
+  | Number n ->
+    let s = Printf.sprintf "%g" n in
+    (* Ensure float-like if no decimal point *)
+    if String.contains s '.' || String.contains s 'e' then s else s
+  | Rational(n, d) -> Printf.sprintf "%d/%d" n d
+  | String s ->
+    let buf = Buffer.create (String.length s + 2) in
+    Buffer.add_char buf '"';
+    String.iter (function
+      | '"'  -> Buffer.add_string buf "\\\""
+      | '\\' -> Buffer.add_string buf "\\\\"
+      | '\n' -> Buffer.add_string buf "\\n"
+      | '\r' -> Buffer.add_string buf "\\r"
+      | '\t' -> Buffer.add_string buf "\\t"
+      | c    -> Buffer.add_char buf c) s;
+    Buffer.add_char buf '"';
+    Buffer.contents buf
+  | Char n ->
+    if n = 32 then "#\\space"
+    else if n = 10 then "#\\newline"
+    else if n = 9 then "#\\tab"
+    else Printf.sprintf "#\\%c" (Char.chr (n land 0xFF))
+  | Symbol s -> s
+  | Keyword k -> ":" ^ k
+  | List items | ListRef { contents = items } ->
+    "(" ^ String.concat " " (List.map sx_write_val items) ^ ")"
+  | v -> inspect v
+
+and sx_display_val = function
+  | String s -> s
+  | Char n -> String.make 1 (Char.chr (n land 0xFF))
+  | v -> sx_write_val v
+
 let () =
   (* === Arithmetic === *)
   register "+" (fun args ->
@@ -2580,3 +2620,71 @@ let () =
       Bool (!pos < String.length src)
     | [Port _] -> Bool false
     | _ -> raise (Eval_error "char-ready?: expected input port"))
+;
+  (* === read / write / display === *)
+  let rec read_postprocess = function
+    | List [] -> Nil
+    | List items -> List (List.map read_postprocess items)
+    | v -> v
+  in
+  register "read" (fun args ->
+    match args with
+    | [] -> Eof
+    | [Port p] ->
+      (match p.sp_kind with
+       | PortOutput _ -> raise (Eval_error "read: expected input port")
+       | PortInput (src, pos) ->
+         let len = String.length src in
+         if p.sp_closed || !pos >= len then Eof
+         else begin
+           let sub = String.sub src !pos (len - !pos) in
+           let s = Sx_parser.make_state sub in
+           Sx_parser.skip_whitespace_and_comments s;
+           if Sx_parser.at_end s then (pos := len; Eof)
+           else
+             (try let form = read_postprocess (Sx_parser.read_value s) in
+                  pos := !pos + s.pos; form
+              with _ -> pos := len; Eof)
+         end)
+    | _ -> raise (Eval_error "read: expected optional input port"));
+  register "write" (fun args ->
+    match args with
+    | [v] -> String (sx_write_val v)
+    | [v; Port p] ->
+      (match p.sp_kind with
+       | PortInput _ -> raise (Eval_error "write: expected output port")
+       | PortOutput buf ->
+         if not p.sp_closed then Buffer.add_string buf (sx_write_val v);
+         Nil)
+    | _ -> raise (Eval_error "write: expected val [port]"));
+  register "display" (fun args ->
+    match args with
+    | [v] -> String (sx_display_val v)
+    | [v; Port p] ->
+      (match p.sp_kind with
+       | PortInput _ -> raise (Eval_error "display: expected output port")
+       | PortOutput buf ->
+         if not p.sp_closed then Buffer.add_string buf (sx_display_val v);
+         Nil)
+    | _ -> raise (Eval_error "display: expected val [port]"));
+  register "newline" (fun args ->
+    match args with
+    | [] -> Nil
+    | [Port p] ->
+      (match p.sp_kind with
+       | PortInput _ -> raise (Eval_error "newline: expected output port")
+       | PortOutput buf ->
+         if not p.sp_closed then Buffer.add_char buf '\n';
+         Nil)
+    | _ -> raise (Eval_error "newline: expected optional output port"));
+  register "write-to-string" (fun args ->
+    match args with
+    | [v] -> String (sx_write_val v)
+    | _ -> raise (Eval_error "write-to-string: 1 arg"));
+  register "display-to-string" (fun args ->
+    match args with
+    | [v] -> String (sx_display_val v)
+    | _ -> raise (Eval_error "display-to-string: 1 arg"));
+  register "current-input-port"  (fun _ -> Nil);
+  register "current-output-port" (fun _ -> Nil);
+  register "current-error-port"  (fun _ -> Nil)