feat(types): tagged-union enums — variant payloads + binding match + exhaustiveness (#56)

Extend `enum` from named int constants to a tagged union: a variant may
carry a payload (`enum Tile { Empty, Wall, Door(int), Portal(int, int) }`).
Such enums box to a heap record (an i32 tag at offset 0, then one 8-byte
slot per payload position); an all-bare enum keeps its zero-cost compile-
time-ordinal representation, byte-for-byte unchanged (every golden render
and the bootstrap fixpoint still hold).

- Parser: variant payload declarations, stored as N_PARAM kids on the
  variant node.
- Construction: by name — `Door(3)`, `Portal(x, y)`, bare `Empty` — resolved
  ahead of the function-call fallback and boxed with the payloads coerced to
  their declared types.
- match: destructures a tagged scrutinee, switching on the tag and binding
  each arm's payload names in a scoped local frame.
- Checking pass: a tagged `match` must be exhaustive (cover every variant or
  end in `_`), and constructor/pattern arities and binding forms are checked
  — all reported where the scrutinee's type is known.

Adds selfhost/tests/enums.ludic to the regression suite and documents the
feature in LANGUAGE.md and the enum/match pages.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

changes/tagged-union-enums.md

@@ -0,0 +1,3 @@
+bump: minor
+type: feat
+Tagged-union enums (#56) — an `enum` variant may now carry a payload (`enum Tile { Empty, Wall, Door(int), Portal(int, int) }`), turning the enum into a sum type. Variants are constructed by name (`Door(3)`, bare `Empty`) and boxed as a tag plus payload slots; `match` destructures them, binding each payload in the arm's scope (`Door(n) => …`, `Portal(x, y) => …`). A tagged `match` is checked for exhaustiveness — it must cover every variant or end in `_`, and constructor/pattern arities are checked too — so adding a variant surfaces every site that must handle it. Plain (all-bare) enums keep their zero-cost compile-time-ordinal `Name.Variant` representation, byte-for-byte unchanged.