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

plans/ocaml-on-sx.md

@@ -407,6 +407,10 @@ _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.
+- 2026-05-08 Phase 2+3 — `try ... with | pat when GUARD -> body` guard
+  support (+3 tests, 467 total). parse-try mirrors match/function;
+  eval-try clause loop now dispatches on `case`/`case-when` and falls
+  through to next clause when guard is false.
 - 2026-05-08 Phase 1+3 — `function | pat when GUARD -> body | …`
   guard support (+3 tests, 464 total). `parse-function` mirrors the
   match-clause when-handling.