refactor!: move raise_all to IteratorExt::raise

[tisonkun]

Summary

• Add IteratorExt::raise(parent) for iterators whose items convert into Exn.
• Replace the public Exn::raise_all entry point with the iterator extension.
• Preserve iteration order, existing exception subtrees, raw-error conversion, empty-iterator behavior, and caller locations.

Why raise

The fluent forms now read symmetrically:

child.raise(parent)
children.into_iter().raise(parent)

The iterator receiver already expresses plurality, so raise_all repeats information. collect_all would suggest collecting Result values and deciding how successes and failures interact.

This intentionally does not revive the fail-slow Iterator<Item = Result<T, E>> design from #32. That is a separate collection operation, comparable to error-stack's try_collect_reports.

Compatibility

This is a breaking API change, recorded in the changelog. The draft keeps one policy question visible for review: remove Exn::raise_all directly for the next release, as implemented here, or deprecate it for one release first.

Validation

• cargo test --workspace
• cargo +nightly clippy --tests --all-features --all-targets --workspace -- -D warnings
• cargo +nightly fmt --all --check
• taplo format --check on tracked TOML files
• typos
• hawkeye check