ocaml: phase 4 'assert EXPR' (+3 tests, 487 total)

Tokenizer already classified 'assert' as a keyword; this commit wires
it through:
  parser  : parse-prefix dispatches like 'not' — advance, recur, wrap
            as (:assert EXPR).
  eval    : evaluate operand; nil on truthy, host-error 'Assert_failure'
            on false. Caught cleanly by existing try/with.

  assert true; 42                          = 42
  let x = 5 in assert (x = 5); x + 1       = 6
  try (assert false; 0) with _ -> 99       = 99

lib/ocaml/eval.sx

@@ -430,6 +430,11 @@
               (else (error (str "ocaml-eval: unbound variable " name))))))
         ((= tag "neg") (- 0 (ocaml-eval (nth ast 1) env)))
         ((= tag "not") (not (ocaml-eval (nth ast 1) env)))
+        ((= tag "assert")
+          (let ((v (ocaml-eval (nth ast 1) env)))
+            (cond
+              ((= v false) (error "Assert_failure"))
+              (else nil))))
         ((= tag "deref")
           (let ((cell (ocaml-eval (nth ast 1) env)))
             (nth cell 0)))

lib/ocaml/parser.sx

@@ -622,6 +622,8 @@
                 (begin (advance-tok!) (list :deref (parse-prefix))))
               ((at-kw? "not")
                 (begin (advance-tok!) (list :not (parse-prefix))))
+              ((at-kw? "assert")
+                (begin (advance-tok!) (list :assert (parse-prefix))))
               (else (parse-app)))))
         (set!
           parse-binop-rhs

lib/ocaml/test.sh

@@ -1202,6 +1202,14 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 4922)
 (eval "(ocaml-run \"let s = \\\"hi\\\" in let n = ref 0 in for i = 0 to String.length s - 1 do n := !n + Char.code s.[i] done; !n\")")
 
+;; ── assert ────────────────────────────────────────────────────
+(epoch 4930)
+(eval "(ocaml-run \"assert true; 42\")")
+(epoch 4931)
+(eval "(ocaml-run \"let x = 5 in assert (x = 5); x + 1\")")
+(epoch 4932)
+(eval "(ocaml-run \"try (assert false; 0) with _ -> 99\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 360 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1906,6 +1914,11 @@ check 4920 "s.[0] hello"                  '"h"'
 check 4921 "Char.code s.[2] abc"          '99'
 check 4922 "for i s.[i] sum hi"           '209'
 
+# ── assert ───────────────────────────────────────────────────────
+check 4930 "assert true; 42"              '42'
+check 4931 "assert (x = 5); x + 1"        '6'
+check 4932 "try (assert false; ...) with" '99'
+
 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.
+- 2026-05-09 Phase 4 — `assert EXPR` (+3 tests, 487 total). Tokenizer
+  already classified `assert` as a keyword; parse-prefix now handles
+  it like `not` (advance, recur, wrap). Eval evaluates the operand and
+  returns nil on truthy, raises `Assert_failure` on false (host-side
+  error so existing try/with handles it). `try (assert false; 0) with
+  _ -> 99` → 99.
 - 2026-05-09 Phase 5.1 — levenshtein.ml baseline (recursive edit
   distance, no memo). Sums distances for five short pairs:
   ("abc","abx")=1 + ("ab","ba")=2 + ("abc","axyc")=2 +