ocaml: phase 2+6 print primitives wire to host display (+2 tests, 444 total)

print_string / print_endline / print_int / print_newline now route to
SX display primitive (not the non-existent print/println). print_endline
appends '\n'.

let _ = expr ;; at top level confirmed working via the
wildcard-param parser.

lib/ocaml/test.sh

@@ -1092,6 +1092,12 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 3802)
 (eval "(ocaml-type-of \"let f x = x + 1 and g x = x * 2 in f 1 + g 2\")")
 
+;; ── let _ = expr top-level ─────────────────────────────────────
+(epoch 3900)
+(eval "(ocaml-run-program \"let _ = 1 + 2 ;; 42\")")
+(epoch 3901)
+(eval "(ocaml-run-program \"let x = 10 ;; let _ = x ;; x * 2\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1728,6 +1734,10 @@ check 3800 "let-mut x+y : Int"            '"Int"'
 check 3801 "let-rec-mut even"             '"Int -> Bool"'
 check 3802 "let-mut f and g"              '"Int"'
 
+# ── let _ = expr top-level ─────────────────────────────────────
+check 3900 "let _ = 1+2;; 42"             '42'
+check 3901 "two top-level lets, _"        '20'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"