ocaml: phase 4 'let open M in body' local opens (+3 tests, 478 total)

Parser detects 'let open' as a separate let-form, parses M as a path
(Ctor(.Ctor)*) directly via inline AST construction (no source slicing
since cur-pos is only available in ocaml-parse-program), and emits
(:let-open PATH BODY).

Eval resolves the path to a module dict and merges its bindings into
the env for body evaluation. Now:

  let open List in map (fun x -> x * 2) [1;2;3]   = [2;4;6]
  let open Option in map (fun x -> x + 1) (Some 5) = Some 6

lib/ocaml/test.sh

@@ -1178,6 +1178,14 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 4801)
 (eval "(ocaml-run-program \"module M = struct let x = 1 and y = 2 end ;; M.x + M.y\")")
 
+;; ── let open M in body ────────────────────────────────────────
+(epoch 4900)
+(eval "(ocaml-run \"let open List in length [1;2;3]\")")
+(epoch 4901)
+(eval "(ocaml-run \"let open List in map (fun x -> x * 2) [1;2;3]\")")
+(epoch 4902)
+(eval "(ocaml-run \"let open Option in map (fun x -> x + 1) (Some 5)\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1867,6 +1875,11 @@ check 4703 "((1+2) : int) * 3"            '9'
 check 4800 "module rec a/b mutual"        '1'
 check 4801 "module x and y"               '3'
 
+# ── let open M in body ─────────────────────────────────────────
+check 4900 "let open List; length"        '3'
+check 4901 "let open List; map"           '(2 4 6)'
+check 4902 "let open Option; map"         '("Some" 6)'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"