spec: string-buffer primitive — make-string-buffer/append!/->string/length

OCaml: StringBuffer of Buffer.t in sx_types.ml; 5 primitives in
sx_primitives.ml (make-string-buffer, string-buffer?, string-buffer-append!,
string-buffer->string, string-buffer-length); inspect case added.

JS: SxStringBuffer with array+join backend; _string_buffer marker for
typeOf dispatch and dict? exclusion (also excludes _vector from dict?).

spec/primitives.sx: 5 define-primitive entries.
17/17 tests pass on both OCaml and JS.

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

hosts/ocaml/lib/sx_primitives.ml

@@ -1368,6 +1368,23 @@ let () =
       if len <= 0 then Vector [||] else Vector (Array.sub arr start len)
     | _ -> raise (Eval_error "vector-copy: expected (vector) or (vector start) or (vector start end)"));
 
+  (* String buffers — O(1) amortised append for string building in loops *)
+  register "make-string-buffer" (fun _ -> StringBuffer (Buffer.create 64));
+  register "string-buffer?" (fun args ->
+    match args with [StringBuffer _] -> Bool true | [_] -> Bool false
+    | _ -> raise (Eval_error "string-buffer?: expected 1 arg"));
+  register "string-buffer-append!" (fun args ->
+    match args with
+    | [StringBuffer buf; String s] -> Buffer.add_string buf s; Nil
+    | [StringBuffer _; v] -> raise (Eval_error ("string-buffer-append!: expected string, got " ^ type_of v))
+    | _ -> raise (Eval_error "string-buffer-append!: expected (buffer string)"));
+  register "string-buffer->string" (fun args ->
+    match args with [StringBuffer buf] -> String (Buffer.contents buf)
+    | _ -> raise (Eval_error "string-buffer->string: expected (buffer)"));
+  register "string-buffer-length" (fun args ->
+    match args with [StringBuffer buf] -> Integer (Buffer.length buf)
+    | _ -> raise (Eval_error "string-buffer-length: expected (buffer)"));
+
   (* Capability-based sandboxing — gate IO operations *)
   let cap_stack : string list ref = ref [] in
   register "with-capabilities" (fun args ->