sx: step 13 — buffer primitives + buffer-based inspect

Added short aliases make-buffer / buffer? / buffer-append! / buffer->string /
buffer-length on both OCaml and JS hosts, sharing the existing StringBuffer
value type. buffer-append! auto-coerces non-strings via inspect.

Rewrote the OCaml host inspect function to walk a single shared Buffer.t
instead of allocating O(n) intermediate strings via String.concat at every
recursion level. inspect underlies sx-serialize and error-path formatting,
so this benefits the tightest serialization paths.

Median improvements (bin/bench_inspect.exe, best-of-3 of 9-run min):
  tree-d8 (75KB):    5.31ms -> 1.30ms  (-76%)
  tree-d10 (679KB): 81.89ms -> 16.02ms (-80%)
  dict-1000:         0.80ms -> 0.31ms  (-61%)
  list-2000:         0.74ms -> 0.33ms  (-55%)

Tests: OCaml 4545 -> 4550. JS 2591 -> 2596. Zero regressions.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

hosts/ocaml/lib/sx_primitives.ml

@@ -1607,6 +1607,32 @@ let () =
     match args with [StringBuffer buf] -> Integer (Buffer.length buf)
     | _ -> raise (Eval_error "string-buffer-length: expected (buffer)"));
 
+  (* Short aliases — same StringBuffer value, terser names for hot paths.
+     Append accepts any value: strings pass through, others get inspected/coerced. *)
+  register "make-buffer" (fun _ -> StringBuffer (Buffer.create 64));
+  register "buffer?" (fun args ->
+    match args with [StringBuffer _] -> Bool true | [_] -> Bool false
+    | _ -> raise (Eval_error "buffer?: expected 1 arg"));
+  register "buffer-append!" (fun args ->
+    match args with
+    | [StringBuffer buf; String s] -> Buffer.add_string buf s; Nil
+    | [StringBuffer buf; Integer n] -> Buffer.add_string buf (string_of_int n); Nil
+    | [StringBuffer buf; Number n] -> Buffer.add_string buf (Sx_types.format_number n); Nil
+    | [StringBuffer buf; Symbol s] -> Buffer.add_string buf s; Nil
+    | [StringBuffer buf; Char n] ->
+      Buffer.add_utf_8_uchar buf (Uchar.of_int n); Nil
+    | [StringBuffer buf; Nil] -> Buffer.add_string buf ""; Nil
+    | [StringBuffer buf; Bool true] -> Buffer.add_string buf "true"; Nil
+    | [StringBuffer buf; Bool false] -> Buffer.add_string buf "false"; Nil
+    | [StringBuffer buf; v] -> Buffer.add_string buf (inspect v); Nil
+    | _ -> raise (Eval_error "buffer-append!: expected (buffer value)"));
+  register "buffer->string" (fun args ->
+    match args with [StringBuffer buf] -> String (Buffer.contents buf)
+    | _ -> raise (Eval_error "buffer->string: expected (buffer)"));
+  register "buffer-length" (fun args ->
+    match args with [StringBuffer buf] -> Integer (Buffer.length buf)
+    | _ -> raise (Eval_error "buffer-length: expected (buffer)"));
+
   (* Capability-based sandboxing — gate IO operations *)
   let cap_stack : string list ref = ref [] in
   register "with-capabilities" (fun args ->