typing: full Maranget usefulness check for ExprKind::Match exhaustiveness #66

Background

PR #65 lands core ExprKind::Match end-to-end across typing, interp, linear/affine, and wasm. ephapax-typing::check_exhaustiveness ships a simplified algorithm that catches the common errors:

• catch-all (wildcard / bare Var) detected
• constructor coverage over the parent data type
• literal-without-wildcard rejected as non-exhaustive

It does not implement full Maranget-style usefulness over nested matrices.

Gaps

1. Nested constructor patterns. match xs with | Cons(_, Nil) -> ... | Cons(_, Cons(_, _)) -> ... | Nil -> ... — typechecker accepts each pattern individually but does not prove the set is exhaustive over the inner column.
2. Literal coverage over infinite domains. For Int / String scrutinees the only way to be exhaustive is a wildcard. We already require this, but we do not yet handle Bool where true + false should be exhaustive without a wildcard.
3. Mixed literal + constructor columns (e.g. matching on a tuple where one slot is a constructor and another is a literal).

Proposal

Port a Maranget specialization-and-default matrix decomposition into ephapax-typing:

• Pattern row → typed column matrix; recurse with specialize(ctor) and default reductions
• usefulness check on the synthetic row of wildcards
• reuse existing DataCtorRegistry for constructor enumeration
• emit NonExhaustiveMatch { missing: pretty(witness) } from the recovered witness pattern

Acceptance

• cargo test -p ephapax-typing --lib covers: bool full coverage without wildcard, nested option-of-option exhaustive, nested option-of-option non-exhaustive (witness reports Some(Some(_)) or similar), tuple of (Option<T>, Bool) exhaustive.
• No regressions in feat(typing,interp,linear,wasm): core ExprKind::Match — typing + interp + N-arm linearity + br_table codegen #65's 11 existing match tests.

References

• PR feat(typing,interp,linear,wasm): core ExprKind::Match — typing + interp + N-arm linearity + br_table codegen #65 — initial simplified implementation
• Maranget, "Warnings for pattern matching" (JFP 2007)