ocaml: phase 4 'M.(expr)' local-open expression form (+3 tests, 481 total)

In parse-atom-postfix, after consuming '.', if the next token is '(',
parse the inner expression and emit (:let-open M EXPR) instead of
:field. Cleanly composes with the existing :let-open evaluator and
loops to allow chained dot postfixes.

  List.(length [1;2;3])                = 3
  List.(map (fun x -> x + 1) [1;2;3])   = [2;3;4]
  Option.(map (fun x -> x * 10) (Some 4)) = Some 40

lib/ocaml/parser.sx

@@ -562,11 +562,22 @@
                     (when (at-op? ".")
                       (begin
                         (advance-tok!)
-                        (let ((tok (peek-tok)))
-                          (begin
-                            (advance-tok!)
-                            (set! head (list :field head (ocaml-tok-value tok)))
-                            (loop)))))))
+                        (cond
+                          ((at-op? "(")
+                            (begin
+                              (advance-tok!)
+                              (let ((inner (parse-expr)))
+                                (begin
+                                  (consume! "op" ")")
+                                  (set! head (list :let-open head inner))
+                                  (loop)))))
+                          (else
+                            (let ((tok (peek-tok)))
+                              (begin
+                                (advance-tok!)
+                                (set! head (list :field head
+                                                 (ocaml-tok-value tok)))
+                                (loop)))))))))
                 (loop)
                 head))))
         (set!

lib/ocaml/test.sh

@@ -1186,6 +1186,14 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 4902)
 (eval "(ocaml-run \"let open Option in map (fun x -> x + 1) (Some 5)\")")
 
+;; ── M.(expr) local-open expression form ───────────────────────
+(epoch 4910)
+(eval "(ocaml-run \"List.(length [1;2;3])\")")
+(epoch 4911)
+(eval "(ocaml-run \"List.(map (fun x -> x + 1) [1;2;3])\")")
+(epoch 4912)
+(eval "(ocaml-run \"Option.(map (fun x -> x * 10) (Some 4))\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1880,6 +1888,11 @@ check 4900 "let open List; length"        '3'
 check 4901 "let open List; map"           '(2 4 6)'
 check 4902 "let open Option; map"         '("Some" 6)'
 
+# ── M.(expr) local-open expression form ──────────────────────────
+check 4910 "M.(expr) length"              '3'
+check 4911 "M.(expr) map"                 '(2 3 4)'
+check 4912 "M.(expr) Option map"          '("Some" 40)'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"

plans/ocaml-on-sx.md

@@ -407,6 +407,12 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-08 Phase 4 — `M.(expr)` local-open expression form (+3
+  tests, 481 total). Implemented in parse-atom-postfix: after
+  consuming `.`, if next token is `(`, parse the inner expression and
+  emit `(:let-open M EXPR)` instead of `:field`. Cleanly composes with
+  existing `:let-open` evaluator. `List.(length [1;2;3])` → 3,
+  `Option.(map (fun x -> x * 10) (Some 4))` → Some 40.
 - 2026-05-08 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)*), and emits `(:let-open PATH BODY)`. Eval