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/bin/bench_inspect.ml

@@ -0,0 +1,46 @@
+(* Benchmark inspect on representative SX values.
+   Takes min of 9 runs of n iterations to dampen GC noise. *)
+open Sx_types
+
+let rec make_tree d =
+  if d = 0 then String "leaf"
+  else List [String "node"; make_tree (d - 1); make_tree (d - 1); make_tree (d - 1)]
+
+let bench_min label f n runs =
+  let times = ref [] in
+  for _ = 1 to runs do
+    Gc.compact ();
+    let t0 = Unix.gettimeofday () in
+    for _ = 1 to n do ignore (f ()) done;
+    let t1 = Unix.gettimeofday () in
+    times := (t1 -. t0) :: !times
+  done;
+  let sorted = List.sort compare !times in
+  let min_t = List.nth sorted 0 in
+  let median = List.nth sorted (runs / 2) in
+  Printf.printf "  %-30s  min=%6.2fms  median=%6.2fms  (n=%d * %d runs)\n%!"
+    label (min_t *. 1000.0 /. float_of_int n)
+    (median *. 1000.0 /. float_of_int n) n runs
+
+let () =
+  let tree8 = make_tree 8 in
+  let s = inspect tree8 in
+  Printf.printf "tree-d8 inspect len=%d\n%!" (String.length s);
+  bench_min "inspect tree-d8" (fun () -> inspect tree8) 50 9;
+
+  let tree10 = make_tree 10 in
+  let s = inspect tree10 in
+  Printf.printf "tree-d10 inspect len=%d\n%!" (String.length s);
+  bench_min "inspect tree-d10" (fun () -> inspect tree10) 5 9;
+
+  let dict_xs = make_dict () in
+  for i = 0 to 999 do
+    Hashtbl.replace dict_xs (string_of_int i) (Integer i)
+  done;
+  let d = Dict dict_xs in
+  bench_min "inspect dict-1000" (fun () -> inspect d) 100 9;
+
+  let xs = ref [] in
+  for i = 0 to 1999 do xs := Integer i :: !xs done;
+  let lst = List !xs in
+  bench_min "inspect list-2000" (fun () -> inspect lst) 200 9

hosts/ocaml/bin/dune

@@ -1,5 +1,5 @@
 (executables
- (names run_tests debug_set sx_server integration_tests bench_cek)
+ (names run_tests debug_set sx_server integration_tests bench_cek bench_inspect)
  (libraries sx unix threads.posix otfm yojson))
 
 (executable