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.

lib/ocaml/eval.sx

@@ -193,6 +193,13 @@
                     (= (len (rest val)) (len arg-pats)))
                 (ocaml-match-list arg-pats (rest val) env))
               (else ocaml-match-fail))))
+        ((= tag "pas")
+          ;; (:pas INNER NAME) — match inner pattern, also bind NAME → val.
+          (let ((inner (nth pat 1)) (alias (nth pat 2)))
+            (let ((env2 (ocaml-match-pat inner val env)))
+              (cond
+                ((= env2 ocaml-match-fail) ocaml-match-fail)
+                (else (ocaml-env-extend env2 alias val))))))
         ((= tag "pcons")
           ;; (:pcons HEAD TAIL) — val must be a non-empty list.
           (cond
@@ -239,11 +246,24 @@
               (error (str "ocaml-eval: match failure on " val)))
             (else
               (let ((clause (first cs)))
-                (let ((pat (nth clause 1)) (body (nth clause 2)))
-                  (let ((env2 (ocaml-match-pat pat val env)))
-                    (cond
-                      ((= env2 ocaml-match-fail) (try-clauses (rest cs)))
-                      (else (ocaml-eval body env2))))))))))
+                (let ((ctag (nth clause 0)))
+                  (cond
+                    ((= ctag "case")
+                      (let ((pat (nth clause 1)) (body (nth clause 2)))
+                        (let ((env2 (ocaml-match-pat pat val env)))
+                          (cond
+                            ((= env2 ocaml-match-fail) (try-clauses (rest cs)))
+                            (else (ocaml-eval body env2))))))
+                    ((= ctag "case-when")
+                      (let ((pat (nth clause 1))
+                            (guard (nth clause 2))
+                            (body (nth clause 3)))
+                        (let ((env2 (ocaml-match-pat pat val env)))
+                          (cond
+                            ((= env2 ocaml-match-fail) (try-clauses (rest cs)))
+                            ((not (ocaml-eval guard env2)) (try-clauses (rest cs)))
+                            (else (ocaml-eval body env2))))))
+                    (else (error (str "ocaml-match: bad clause tag " ctag))))))))))
       (try-clauses clauses))))
 
 (define ocaml-match-eval