chore: prepare exn v0.4.0-rc.1

[tisonkun]

Summary

• bump only exn to 0.4.0-rc.1; keep exn-anyhow at its independently released 0.1.0
• turn the unreleased notes into complete RC release notes, including both expected breaking changes and the exact ?Sized scope
• document the recommended typed module API → erased callback boundary → typed parent workflow
• add concise migration guidance for IteratorExt::raise and the removed ResultExt::Error

Release plan

This PR prepares the release but does not publish or tag it.

After merge:

1. Run cargo release --package exn --prev-tag-name v0.3.1 --execute to publish only exn and create the signed v0.4.0-rc.1 tag. A GitHub Release is not required.
2. Replace the vendored/forked error implementation in gitoxide with the RC and validate the real callback/delegate workflow.
3. Address any integration findings, then prepare exn v0.4.0. exn-anyhow remains on its own release cadence.

The cargo-release dry run explicitly skips exn-anyhow.

API audit

cargo-semver-checks against 0.3.1 reports exactly the two intentional breaking changes documented here:

• removal of Exn::raise_all
• removal of ResultExt::Error

There are currently no open issues. #35 is covered by #60, #61, and #62.

Validation

• cargo x lint
• cargo x test --no-capture
• cargo package --package exn --allow-dirty
• cargo release --package exn --prev-tag-name v0.3.1 --verbose (dry run)