ocaml: string ports (Eof + Port variants, 15 primitives)

Eof sentinel and Port{PortInput/PortOutput} in sx_types.ml. All 15 port
primitives in sx_primitives.ml. type_of/inspect updated. 39/39 port tests
pass (4532 total, +39, zero regressions).

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

hosts/ocaml/lib/sx_primitives.ml

@@ -2301,4 +2301,109 @@ let () =
         | v -> raise (Eval_error ("list->string: expected char, got " ^ type_of v))
       ) chars;
       String (Buffer.contents buf)
-    | _ -> raise (Eval_error "list->string: expected list of chars"))
+    | _ -> raise (Eval_error "list->string: expected list of chars"));
+  (* Phase 14 — EOF object + string ports *)
+  register "eof-object" (fun _args -> Eof);
+  register "eof-object?" (fun args ->
+    match args with
+    | [Eof] -> Bool true
+    | [_]   -> Bool false
+    | _ -> raise (Eval_error "eof-object?: expected 1 argument"));
+  register "open-input-string" (fun args ->
+    match args with
+    | [String s] ->
+      Port { sp_closed = false; sp_kind = PortInput (s, ref 0) }
+    | _ -> raise (Eval_error "open-input-string: expected string"));
+  register "open-output-string" (fun args ->
+    match args with
+    | [] -> Port { sp_closed = false; sp_kind = PortOutput (Buffer.create 64) }
+    | _ -> raise (Eval_error "open-output-string: expected no arguments"));
+  register "get-output-string" (fun args ->
+    match args with
+    | [Port { sp_kind = PortOutput buf; _ }] -> String (Buffer.contents buf)
+    | _ -> raise (Eval_error "get-output-string: expected output port"));
+  register "port?" (fun args ->
+    match args with
+    | [Port _] -> Bool true
+    | [_]      -> Bool false
+    | _ -> raise (Eval_error "port?: expected 1 argument"));
+  register "input-port?" (fun args ->
+    match args with
+    | [Port { sp_kind = PortInput _; _ }] -> Bool true
+    | [_]                                  -> Bool false
+    | _ -> raise (Eval_error "input-port?: expected 1 argument"));
+  register "output-port?" (fun args ->
+    match args with
+    | [Port { sp_kind = PortOutput _; _ }] -> Bool true
+    | [_]                                   -> Bool false
+    | _ -> raise (Eval_error "output-port?: expected 1 argument"));
+  register "close-port" (fun args ->
+    match args with
+    | [Port p] -> p.sp_closed <- true; Nil
+    | _ -> raise (Eval_error "close-port: expected port"));
+  register "read-char" (fun args ->
+    match args with
+    | [] -> raise (Eval_error "read-char: no default port in this host")
+    | [Port p] ->
+      (match p.sp_kind with
+       | PortOutput _ -> raise (Eval_error "read-char: expected input port")
+       | PortInput (src, pos) ->
+         if p.sp_closed || !pos >= String.length src then Eof
+         else begin
+           let cp = Char.code src.[!pos] in
+           incr pos;
+           Char cp
+         end)
+    | _ -> raise (Eval_error "read-char: expected input port"));
+  register "peek-char" (fun args ->
+    match args with
+    | [] -> raise (Eval_error "peek-char: no default port in this host")
+    | [Port p] ->
+      (match p.sp_kind with
+       | PortOutput _ -> raise (Eval_error "peek-char: expected input port")
+       | PortInput (src, pos) ->
+         if p.sp_closed || !pos >= String.length src then Eof
+         else Char (Char.code src.[!pos]))
+    | _ -> raise (Eval_error "peek-char: expected input port"));
+  register "read-line" (fun args ->
+    match args with
+    | [] -> raise (Eval_error "read-line: no default port in this host")
+    | [Port p] ->
+      (match p.sp_kind with
+       | PortOutput _ -> raise (Eval_error "read-line: expected input port")
+       | PortInput (src, pos) ->
+         if p.sp_closed || !pos >= String.length src then Eof
+         else begin
+           let start = !pos in
+           let len = String.length src in
+           while !pos < len && src.[!pos] <> '\n' do incr pos done;
+           let line = String.sub src start (!pos - start) in
+           if !pos < len then incr pos;
+           String line
+         end)
+    | _ -> raise (Eval_error "read-line: expected input port"));
+  register "write-char" (fun args ->
+    match args with
+    | [Char n; Port p] ->
+      (match p.sp_kind with
+       | PortInput _ -> raise (Eval_error "write-char: expected output port")
+       | PortOutput buf ->
+         if not p.sp_closed then
+           Buffer.add_char buf (Char.chr (n land 0xFF));
+         Nil)
+    | _ -> raise (Eval_error "write-char: expected char and output port"));
+  register "write-string" (fun args ->
+    match args with
+    | [String s; Port p] ->
+      (match p.sp_kind with
+       | PortInput _ -> raise (Eval_error "write-string: expected output port")
+       | PortOutput buf ->
+         if not p.sp_closed then Buffer.add_string buf s;
+         Nil)
+    | _ -> raise (Eval_error "write-string: expected string and output port"));
+  register "char-ready?" (fun args ->
+    match args with
+    | [Port { sp_closed = false; sp_kind = PortInput (src, pos); _ }] ->
+      Bool (!pos < String.length src)
+    | [Port _] -> Bool false
+    | _ -> raise (Eval_error "char-ready?: expected input port"))