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_parser.ml

@@ -89,8 +89,18 @@ let read_symbol s =
   while s.pos < s.len && is_symbol_char s.src.[s.pos] do advance s done;
   String.sub s.src start (s.pos - start)
 
+let gcd a b =
+  let rec g a b = if b = 0 then a else g b (a mod b) in g (abs a) (abs b)
+
+let make_rat n d =
+  if d = 0 then raise (Parse_error "rational: division by zero");
+  let sign = if d < 0 then -1 else 1 in
+  let g = gcd (abs n) (abs d) in
+  let rn = sign * n / g and rd = sign * d / g in
+  if rd = 1 then Integer rn else Rational (rn, rd)
+
 let try_number str =
-  (* Integers (no '.' or 'e'/'E') → exact Integer; floats → inexact Number *)
+  (* Integers (no '.' or 'e'/'E') → exact Integer; rationals N/D; floats → inexact Number *)
   let has_dec = String.contains str '.' in
   let has_exp = String.contains str 'e' || String.contains str 'E' in
   if has_dec || has_exp then
@@ -98,13 +108,19 @@ let try_number str =
     | Some n -> Some (Number n)
     | None -> None
   else
-    match int_of_string_opt str with
-    | Some n -> Some (Integer n)
-    | None ->
-      (* handles "nan", "inf", "-inf" *)
-      match float_of_string_opt str with
-      | Some n -> Some (Number n)
-      | None -> None
+    match String.split_on_char '/' str with
+    | [num_s; den_s] when num_s <> "" && den_s <> "" ->
+      (match int_of_string_opt num_s, int_of_string_opt den_s with
+       | Some n, Some d -> (try Some (make_rat n d) with _ -> None)
+       | _ -> None)
+    | _ ->
+      match int_of_string_opt str with
+      | Some n -> Some (Integer n)
+      | None ->
+        (* handles "nan", "inf", "-inf" *)
+        match float_of_string_opt str with
+        | Some n -> Some (Number n)
+        | None -> None
 
 let rec read_value s : value =
   skip_whitespace_and_comments s;
@@ -141,6 +157,13 @@ let rec read_value s : value =
       advance s;
       Char (Char.code c)
     end
+  | '#' when s.pos + 1 < s.len &&
+             (s.src.[s.pos + 1] = 't' || s.src.[s.pos + 1] = 'f') &&
+             (s.pos + 2 >= s.len || not (is_ident_char s.src.[s.pos + 2])) ->
+    (* #t / #f — boolean literals (R7RS shorthand) *)
+    let b = s.src.[s.pos + 1] = 't' in
+    advance s; advance s;
+    Bool b
   | '#' when s.pos + 1 < s.len && s.src.[s.pos + 1] = ';' ->
     (* Datum comment: #; discards next expression *)
     advance s; advance s;