ocaml: phase 3 'as' alias + 'when' guard in match (+6 tests, 295 total)

Pattern parser top wraps cons-pat with 'as ident' -> (:pas PAT NAME).
Match clause parser consumes optional 'when GUARD-EXPR' before -> and
emits (:case-when PAT GUARD BODY) instead of :case.

Eval: :pas matches inner pattern then binds the alias name; case-when
checks the guard after a successful match and falls through to the next
clause if the guard is false.

Or-patterns deferred — ambiguous with clause separator without
parens-only support.

plans/ocaml-on-sx.md

@@ -136,9 +136,11 @@ SX CEK evaluator (both JS and OCaml hosts)
       bindings.)_
 - [~] **Patterns:** constructor (nullary + with args, incl. flattened tuple
       args `Pair (a, b)` → `(:pcon "Pair" PA PB)`), literal (int/string/char/
-      bool/unit), variable, wildcard `_`, tuple, list cons `::`, list literal.
-      _(Pending: record patterns, `as` binding, or-pattern `P1 | P2`, `when`
-      guard.)_
+      bool/unit), variable, wildcard `_`, tuple, list cons `::`, list
+      literal, `as` binding (`pat as name`). Match clauses support `when`
+      guard via `(:case-when PAT GUARD BODY)`. _(Pending: record patterns,
+      or-pattern `P1 | P2` — ambiguous with clause separator without
+      lookahead.)_
 - [ ] OCaml is **not** indentation-sensitive — no layout algorithm needed.
 - [ ] Tests in `lib/ocaml/tests/parse.sx` — 50+ round-trip parse tests.
 
@@ -355,6 +357,14 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 3 — `as` aliases + `when` guards in match (+6 tests,
+  295 total). Parser: pattern parser wraps with `as ident` → `(:pas
+  PAT NAME)`. Match's `one` consumes optional `when GUARD-EXPR` → emits
+  `(:case-when PAT GUARD BODY)` instead of `:case`. Eval `:pas` matches
+  inner pattern then also binds the alias name; `case-when` checks the
+  guard after a successful match and falls through if false. Or-pat
+  `(P1 | P2)` deferred — ambiguous with clause separator without
+  parens-only support.
 - 2026-05-08 Phase 1+2 — record literals `{ x = 1; y = 2 }` and
   functional update `{ r with x = 99 }`. Parser produces `(:record (F E)
   ...)` and `(:record-update BASE-EXPR (F E) ...)`. Eval builds a dict