ocaml: phase 4 'lazy EXPR' + Lazy.force (+2 tests, 496 total)

Tokenizer already had 'lazy' as a keyword. This commit wires it through:

  parser  : parse-prefix emits (:lazy EXPR), like the existing 'assert'
            handler.
  eval    : creates a one-element cell with state ('Thunk' expr env).
  host    : _lazy_force flips the cell to ('Forced' v) on first call
            and returns the cached value thereafter.
  runtime : module Lazy = struct let force lz = _lazy_force lz end.

Memoisation confirmed by tracking a side-effect counter through two
forces of the same lazy:

  let counter = ref 0 in
  let lz = lazy (counter := !counter + 1; 42) in
  let a = Lazy.force lz in
  let b = Lazy.force lz in
  (a + b) * 100 + !counter        = 8401   (= 84*100 + 1)

lib/ocaml/test.sh

@@ -1228,6 +1228,12 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 4951)
 (eval "(ocaml-run \"let t = Hashtbl.create 4 in Hashtbl.add t \\\"x\\\" 10; Hashtbl.add t \\\"y\\\" 20; let total = ref 0 in Hashtbl.iter (fun _ v -> total := !total + v) t; !total\")")
 
+;; ── lazy / Lazy.force ─────────────────────────────────────────
+(epoch 4960)
+(eval "(ocaml-run \"let x = lazy (1 + 2) in Lazy.force x\")")
+(epoch 4961)
+(eval "(ocaml-run \"let counter = ref 0 in let lz = lazy (counter := !counter + 1; 42) in let a = Lazy.force lz in let b = Lazy.force lz in (a + b) * 100 + !counter\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 360 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1948,6 +1954,10 @@ check 4944 "string_of_int + string_of_b"  '"7-true"'
 check 4950 "Hashtbl.fold sum 1+2+3"       '6'
 check 4951 "Hashtbl.iter ref accum 10+20" '30'
 
+# ── lazy / Lazy.force ─────────────────────────────────────────
+check 4960 "lazy 1+2 force"               '3'
+check 4961 "lazy memoization counter=1"   '8401'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"