ocaml: phase 1+3 'when' guard in 'function | pat -> body' (+3 tests, 464 total)

parse-function now consumes optional 'when GUARD-EXPR' before -> and
emits (:case-when PAT GUARD BODY) — same handling as match clauses.
function-style sign extraction now works:
  (function | n when n > 0 -> 1 | n when n < 0 -> -1 | _ -> 0)

lib/ocaml/test.sh

@@ -1140,6 +1140,14 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 4306)
 (eval "(ocaml-run \"Char.is_lower \\\"a\\\"\")")
 
+;; ── function with `when` guard ────────────────────────────────
+(epoch 4400)
+(eval "(ocaml-run \"(function | n when n > 0 -> 1 | _ -> 0) 5\")")
+(epoch 4401)
+(eval "(ocaml-run \"(function | n when n > 0 -> 1 | _ -> 0) (-3)\")")
+(epoch 4402)
+(eval "(ocaml-run \"(function | n when n > 0 -> 1 | n when n < 0 -> -1 | _ -> 0) 0\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1805,6 +1813,11 @@ check 4304 "Char.is_whitespace ' '"       'true'
 check 4305 "Char.is_upper A"              'true'
 check 4306 "Char.is_lower a"              'true'
 
+# ── function with `when` guard ─────────────────────────────────
+check 4400 "function when 5"              '1'
+check 4401 "function when -3"             '0'
+check 4402 "function sign 0"              '0'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"