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>

tools/x/selfhost_test.ludic

@@ -86,6 +86,7 @@ function cmd_selfhost_test() -> int {
   sh_case("sort", "0 39 20 1 2 3 3 1 30 10 20 40 2 8")
   sh_case("control", "55 4 15 1")
   sh_case("match_bits", "1 2 9 16 4 15")
+  sh_case("enums", "1 0 15 7 19 18 0 80 17")
   sh_case("fixed", "2 3 0 6 1")
   sh_case("const", "1 0 10 5 20 0 10 7 1 0")
   sh_case("math", "3 7 5 10 -1 0 1 2 3 3 2 5 25 50")