fix(rt): closure aggregate + #setTupleArgs fallback

[Stevengre]

Summary

Add a reduction rule for aggregateKindClosure and an [owise] fallback for #setTupleArgs when the argument is a plain Value (not wrapped in a tuple Aggregate).

• Add aggregateKindClosure rule: ARGS:List ~> #mkAggregate(aggregateKindClosure(...)) => Aggregate(variantIdx(0), ARGS)
• Add #setTupleArgs(IDX, VAL:Value) [owise] fallback for single-value arguments
• Add closure-no-capture.rs regression test

Context

MIR represents closure construction via Rvalue::Aggregate(AggregateKind::Closure(...)). The K semantics had rules for aggregateKindTuple and aggregateKindAdt but no rule for aggregateKindClosure, leaving the term stuck after collecting operands into a List.

Additionally, when a closure is called with a single argument, the Rust calling convention passes it as a plain Value rather than wrapping it in a tuple Aggregate. The existing #setTupleArgs rules only matched Aggregate(variantIdx(0), ...) or List — the [owise] fallback handles the unwrapped case.

This matches miri's closure handling where closure aggregates are constructed the same way as struct aggregates, and single-element tuples are not wrapped.

Without this fix, any program constructing a closure gets stuck:

Leaf <k>:
  ListItem(...) ~> #mkAggregate(aggregateKindClosure(...))
  -- no matching rule, term is stuck

With this fix, closure construction reduces to Aggregate(variantIdx(0), ARGS) and single-value tuple-arg dispatch works correctly.

Proof evidence

Without fix (RED): closure-no-capture.rs gets stuck at #mkAggregate(aggregateKindClosure(...))

With fix (GREEN):

test_prove_rs[closure-no-capture] PASSED

Test plan

• test_prove_rs[closure-no-capture]
• make test-integration regression