ocaml: phase 4 basic labeled / optional argument syntax (label dropped) (+3 tests, 562 total)

Three parser changes:

  1. at-app-start? returns true on op '~' or '?' so the app loop
     keeps consuming labeled args.
  2. The app arg parser handles:
       ~name:VAL    drop label, parse VAL as the arg
       ?name:VAL    same
       ~name        punning -- treat as (:var name)
       ?name        same
  3. try-consume-param! drops '~' or '?' and treats the following
     ident as a regular positional param name.

Caveats:
  - Order in the call must match definition order; we don't reorder
    by label name.
  - Optional args don't auto-wrap in Some, so the function body sees
    the raw value for ?x:V.

Lets us write idiomatic-looking OCaml even though the runtime is
positional underneath:

  let f ~x ~y = x + y in f ~x:3 ~y:7         = 10
  let x = 4 in let y = 5 in f ~x ~y          = 20  (punning)
  let f ?x ~y = x + y in f ?x:1 ~y:2         = 3

lib/ocaml/test.sh

@@ -1396,6 +1396,14 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 5142)
 (eval "(ocaml-run-program \"let h (a, b) (c, d) = a * b + c * d ;; h (1, 2) (3, 4)\")")
 
+;; ── Labeled / optional args (label dropped) ──────────────────
+(epoch 5150)
+(eval "(ocaml-run \"let f ~x ~y = x + y in f ~x:3 ~y:7\")")
+(epoch 5151)
+(eval "(ocaml-run \"let f ~x ~y = x * y in let x = 4 in let y = 5 in f ~x ~y\")")
+(epoch 5152)
+(eval "(ocaml-run \"let f ?x ~y = x + y in f ?x:1 ~y:2\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 360 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -2219,6 +2227,11 @@ check 5140 "top let f (a, b) = a+b"       '10'
 check 5141 "top let g x (a, b) mixed"     '6'
 check 5142 "top let h (a, b) (c, d)"      '14'
 
+# ── Labeled / optional args (label dropped, positional) ─────────
+check 5150 "f ~x:3 ~y:7 sum"              '10'
+check 5151 "f ~x ~y punning"              '20'
+check 5152 "f ?x:1 ~y:2 (no Some wrap)"   '3'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"