refactor: make bare Exn the erased type

[andylokandy]

Stacked on #61.

Summary

• make bare Exn default to dyn Error + Send + Sync
• convert plain errors and typed Exn<E> into bare Exn, allowing ? at erased boundaries
• simplify IteratorExt::raise to erase child markers through Into<Exn>
• remove the separate ErasedExn alias and Exn::erase

Design Notes

In a type position, the default parameter makes Into<Exn> equivalent to Into<Exn<dyn Error + Send + Sync + 'static>>.

Concrete errors and Exn<E> values with sized root markers convert into bare Exn, so they work directly with IteratorExt::raise. Bare Exn values use the standard identity conversion. Together these cover the supported typed and erased states.

The ?Sized marker bounds allow the standard erased dyn Error + Send + Sync default without reallocating or changing the exception tree. This PR does not provide conversion to arbitrary custom trait-object markers such as Exn<dyn CustomError>.

Validation

• cargo x lint
• cargo x test --no-capture
• integration coverage for automatic callback erasure, typed and erased iterator items, allocation reuse, and runtime downcasting