ocaml: phase 2+3 'when' guard in try/with (+3 tests, 467 total)

parse-try now consumes optional 'when GUARD-EXPR' before -> and emits
(:case-when PAT GUARD BODY). Eval try clause loop dispatches on case /
case-when and falls through on guard false — same semantics as match.

Examples:
  try raise (E 5) with | E n when n > 0 -> n | _ -> 0   = 5
  try raise (E (-3)) with | E n when n > 0 -> n | _ -> 0 = 0
  try raise (E 5) with | E n when n > 100 -> n | E n -> n + 1000  = 1005

lib/ocaml/test.sh

@@ -1148,6 +1148,14 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 4402)
 (eval "(ocaml-run \"(function | n when n > 0 -> 1 | n when n < 0 -> -1 | _ -> 0) 0\")")
 
+;; ── try/with `when` guard ─────────────────────────────────────
+(epoch 4500)
+(eval "(ocaml-run \"try raise (E 5) with | E n when n > 0 -> n | _ -> 0\")")
+(epoch 4501)
+(eval "(ocaml-run \"try raise (E (-3)) with | E n when n > 0 -> n | _ -> 0\")")
+(epoch 4502)
+(eval "(ocaml-run \"try raise (E 5) with | E n when n > 100 -> n | E n -> n + 1000\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1818,6 +1826,11 @@ check 4400 "function when 5"              '1'
 check 4401 "function when -3"             '0'
 check 4402 "function sign 0"              '0'
 
+# ── try/with `when` guard ──────────────────────────────────────
+check 4500 "try when guard fires"         '5'
+check 4501 "try when guard skips"         '0'
+check 4502 "try when fall through"        '1005'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"