ocaml: phase 1+6 Buffer + parser !x in app args (+3 tests, 425 total)

Parser fix: at-app-start? and parse-app's loop recognise prefix !
as a deref of the next app arg. So 'List.rev !b' parses as
'(:app List.rev (:deref b))' instead of stalling at !.

Buffer module backed by a ref holding string list:
  create _ = ref []
  add_string b s = b := s :: !b
  contents b = String.concat "" (List.rev !b)
  add_char/length/clear/reset

lib/ocaml/test.sh

@@ -1044,6 +1044,14 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 3403)
 (eval "(ocaml-run-program \"module IntOrd = struct let compare a b = compare a b end ;; module IntSet = Set.Make(IntOrd) ;; let a = IntSet.add 1 (IntSet.add 2 (IntSet.add 3 IntSet.empty)) ;; let b = IntSet.add 2 (IntSet.add 3 (IntSet.add 4 IntSet.empty)) ;; IntSet.elements (IntSet.inter a b)\")")
 
+;; ── Buffer module ──────────────────────────────────────────────
+(epoch 3500)
+(eval "(ocaml-run-program \"let b = Buffer.create 16 ;; Buffer.add_string b \\\"Hello\\\" ;; Buffer.add_string b \\\", \\\" ;; Buffer.add_string b \\\"World\\\" ;; Buffer.contents b\")")
+(epoch 3501)
+(eval "(ocaml-run-program \"let b = Buffer.create 16 ;; Buffer.add_string b \\\"abc\\\" ;; Buffer.length b\")")
+(epoch 3502)
+(eval "(ocaml-run-program \"let b = Buffer.create 16 ;; Buffer.add_string b \\\"x\\\" ;; Buffer.clear b ;; Buffer.contents b\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1652,6 +1660,11 @@ check 3401 "Map.is_empty false"           'false'
 check 3402 "Set.union"                    '(1 2 3)'
 check 3403 "Set.inter"                    '(2 3)'
 
+# ── Buffer module ──────────────────────────────────────────────
+check 3500 "Buffer concat 'Hello, World'" '"Hello, World"'
+check 3501 "Buffer.length 3"              '3'
+check 3502 "Buffer.clear empties"         '""'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"