ocaml: phase 2 mutable record fields r.f <- v (+4 tests, 451 total)

<- added to op-table at level 1 (same as :=). Eval short-circuits on
<- to mutate the lhs's field via host SX dict-set!. The lhs must be a
:field expression; otherwise raises.

Tested:
  let r = { x = 1; y = 2 } in r.x <- 5; r.x       (5)
  let r = { x = 0 } in for i = 1 to 5 do r.x <- r.x + i done; r.x  (15)
  let r = { name = ...; age = 30 } in r.name <- "Alice"; r.name

The 'mutable' keyword in record type decls is parsed-and-discarded;
runtime semantics: every field is mutable. Phase 2 closes this gap
without changing the dict-based record representation.

lib/ocaml/test.sh

@@ -1106,6 +1106,16 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 4002)
 (eval "(ocaml-run-program \"type point = { x : int; y : int };; let p = { x = 3; y = 4 };; p.x + p.y\")")
 
+;; ── Mutable record fields (r.f <- v) ──────────────────────────
+(epoch 4100)
+(eval "(ocaml-run \"let r = { x = 1; y = 2 } in r.x <- 5; r.x\")")
+(epoch 4101)
+(eval "(ocaml-run \"let r = { x = 0 } in for i = 1 to 5 do r.x <- r.x + i done; r.x\")")
+(epoch 4102)
+(eval "(ocaml-run \"let r = { name = \\\"Bob\\\"; age = 30 } in r.name <- \\\"Alice\\\"; r.name\")")
+(epoch 4103)
+(eval "(ocaml-run \"let r = { x = 1; y = 2 } in r.x <- r.y * 10; r.x\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1751,6 +1761,12 @@ check 4000 "record type decl"             '("type-def-record" "point" () (("x")
 check 4001 "mutable field decl"           '("mutable" "x")'
 check 4002 "record decl + use"            '7'
 
+# ── Mutable record fields ──────────────────────────────────────
+check 4100 "r.x <- 5; r.x"                '5'
+check 4101 "for-loop accum r.x"           '15'
+check 4102 "r.name <- str"                '"Alice"'
+check 4103 "r.x <- r.y * 10"              '20'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"