ocaml: phase 3 pattern matching + constructors (+18 tests, 183 total)

Constructor app: (:app (:con NAME) arg) -> (NAME …args). Tuple args
flatten so Pair(a,b) -> ("Pair" a b), matching the parser's pattern
flatten. Standalone (:con NAME) -> (NAME) nullary.

Pattern matcher: :pwild, :pvar, :plit, :pcon (head + arity), :pcons
(decompose), :plist (length match), :ptuple (after tuple tag). Match
walks clauses until first success; runtime error on exhaustion.
Recursive list functions (len, sum, fact) work end-to-end.

plans/ocaml-on-sx.md

@@ -162,10 +162,13 @@ SX CEK evaluator (both JS and OCaml hosts)
 ### Phase 3 — ADTs + pattern matching
 
 - [ ] `type` declarations: `type t = A | B of t1 * t2 | C of { x: int }`.
-- [ ] Constructors as tagged lists: `A` → `(:A)`, `B(1, "x")` → `(:B 1 "x")`.
-- [ ] `match`/`with`: constructor, literal, variable, wildcard, tuple, list cons/nil,
-      `as` binding, or-patterns, nested patterns, `when` guard.
-- [ ] Exhaustiveness: runtime error on incomplete match (no compile-time check yet).
+      _(Parser + evaluator currently inferred-arity at runtime; type decls
+      pending.)_
+- [x] Constructors as tagged lists: `A` → `("A")`, `B(1, "x")` → `("B" 1 "x")`.
+- [~] `match`/`with`: constructor, literal, variable, wildcard, tuple, list
+      cons/nil, nested patterns. _(Pending: `as` binding, or-patterns,
+      `when` guard.)_
+- [x] Exhaustiveness: runtime error on incomplete match (no compile-time check yet).
 - [ ] Built-in types: `option` (`None`/`Some`), `result` (`Ok`/`Error`),
       `list` (nil/cons), `bool`, `unit`, `exn`.
 - [ ] `exception` declarations; built-in: `Not_found`, `Invalid_argument`,
@@ -317,6 +320,20 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 3 — pattern matching evaluator + constructors (+18
+  tests, 183 total). Constructor application: `(:app (:con NAME) arg)`
+  builds a tagged list `(NAME …args)` with tuple args flattened (so
+  `Pair (a, b)` → `("Pair" a b)` matches the parser's pattern flatten).
+  Standalone ctor `(:con NAME)` → `(NAME)` (nullary). Pattern matcher:
+  :pwild / :pvar / :plit (unboxed compare) / :pcon (head + arity match) /
+  :pcons (cons-decompose) / :plist (length+items) / :ptuple (after `tuple`
+  tag). Match drives clauses until first success; runtime error on
+  exhaustion. Tested with option match, literal match, tuple match,
+  recursive list functions (`len`, `sum`), nested ctor (`Pair(a,b)`).
+  Note: arity flattening happens for any tuple-arg ctor — without ADT
+  declarations there's no way to distinguish `Some (1,2)` (single tuple
+  payload) from `Pair (1,2)` (two-arg ctor). All-flatten convention is
+  consistent across parser + evaluator.
 - 2026-05-08 Phase 2 — `lib/ocaml/eval.sx`: ocaml-eval + ocaml-run +
   ocaml-run-program. Coverage: atoms, var lookup, :app (curried),
   :op (arithmetic/comparison/boolean/^/mod/::/|>), :neg, :not, :if,