ocaml: phase 6 String extensions (+6 tests, 406 total)

ends_with, contains, trim, split_on_char, replace_all, index_of —
wrap host SX primitives via new _string_* builtins. String module
now substantively covers OCaml's Stdlib.String.

lib/ocaml/test.sh

@@ -990,6 +990,20 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 2905)
 (eval "(ocaml-run \"List.take 100 [1;2;3]\")")
 
+;; ── String extensions ──────────────────────────────────────────
+(epoch 3000)
+(eval "(ocaml-run \"String.ends_with \\\"lo\\\" \\\"hello\\\"\")")
+(epoch 3001)
+(eval "(ocaml-run \"String.contains \\\"hello\\\" \\\"ell\\\"\")")
+(epoch 3002)
+(eval "(ocaml-run \"String.trim \\\"  hi  \\\"\")")
+(epoch 3003)
+(eval "(ocaml-run \"String.split_on_char \\\" \\\" \\\"a b c\\\"\")")
+(epoch 3004)
+(eval "(ocaml-run \"String.replace_all \\\"hello\\\" \\\"l\\\" \\\"r\\\"\")")
+(epoch 3005)
+(eval "(ocaml-run \"String.index_of \\\"hello\\\" \\\"ll\\\"\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1566,6 +1580,14 @@ check 2903 "List.flat_map double"         '(1 1 2 2 3 3)'
 check 2904 "List.take 0"                  '()'
 check 2905 "List.take overflow"           '(1 2 3)'
 
+# ── String extensions ──────────────────────────────────────────
+check 3000 "String.ends_with"             'true'
+check 3001 "String.contains"              'true'
+check 3002 "String.trim"                  '"hi"'
+check 3003 "String.split_on_char"         '("a" "b" "c")'
+check 3004 "String.replace_all"           '"herro"'
+check 3005 "String.index_of"              '2'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"