ocaml: phase 6 Printf.sprintf %d/%s/%f/%c/%b/%% + global string_of_* (+5 tests, 492 total)

Replaces the stub sprintf in runtime.sx with a real implementation:
walk fmt char-by-char accumulating a prefix; on recognised %X return a
one-arg fn that formats the arg and recurses on the rest of fmt. The
function self-curries to the spec count — there's no separate arity
machinery, just a closure chain.

Specs: %d (int), %s (string), %f (float), %c (char/string in our model),
%b (bool), %% (literal). Unknown specs pass through.

Same expression returns a string (no specs) or a function (>=1 spec) —
OCaml proper would reject this; works fine in OCaml-on-SX's dynamic
runtime.

Also adds top-level aliases:
  string_of_int   = _string_of_int
  string_of_float = _string_of_float
  string_of_bool  = if b then "true" else "false"
  int_of_string   = _int_of_string

  Printf.sprintf "x=%d" 42              = "x=42"
  Printf.sprintf "%s = %d" "answer" 42  = "answer = 42"
  Printf.sprintf "%d%%" 50               = "50%"

lib/ocaml/test.sh

@@ -1210,6 +1210,18 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 4932)
 (eval "(ocaml-run \"try (assert false; 0) with _ -> 99\")")
 
+;; ── Printf.sprintf + global string_of_* ──────────────────────
+(epoch 4940)
+(eval "(ocaml-run \"Printf.sprintf \\\"hello\\\"\")")
+(epoch 4941)
+(eval "(ocaml-run \"Printf.sprintf \\\"x=%d\\\" 42\")")
+(epoch 4942)
+(eval "(ocaml-run \"Printf.sprintf \\\"%s = %d\\\" \\\"answer\\\" 42\")")
+(epoch 4943)
+(eval "(ocaml-run \"Printf.sprintf \\\"%d%%\\\" 50\")")
+(epoch 4944)
+(eval "(ocaml-run \"string_of_int 7 ^ \\\"-\\\" ^ string_of_bool true\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 360 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1919,6 +1931,13 @@ check 4930 "assert true; 42"              '42'
 check 4931 "assert (x = 5); x + 1"        '6'
 check 4932 "try (assert false; ...) with" '99'
 
+# ── Printf.sprintf ───────────────────────────────────────────────
+check 4940 "sprintf no args"              '"hello"'
+check 4941 "sprintf one %d"               '"x=42"'
+check 4942 "sprintf %s = %d"              '"answer = 42"'
+check 4943 "sprintf %d%% literal percent" '"50%"'
+check 4944 "string_of_int + string_of_b"  '"7-true"'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"