chore: release v0.3.1

[andylokandy]

Summary

• Release exn v0.3.1.
• Document the compact ASCII Debug output change.
• Document the boxed std::error::Error conversion fix.
• Bump exn version references in the manifests and lockfile.

[Copilot]

Pull request overview

Releases exn v0.3.1 by bumping crate/workspace version references and documenting the release notes in the changelog.

Changes:

• Bump exn version to 0.3.1 in the crate manifest and workspace dependency entry.
• Update Cargo.lock to reflect the new exn package version.
• Add a v0.3.1 changelog entry documenting the Debug output formatting change and the boxed std::error::Error conversion fix.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
exn/Cargo.toml	Bumps the exn crate version to 0.3.1.
Cargo.toml	Updates the workspace dependency version reference for exn to 0.3.1.
Cargo.lock	Updates the locked workspace package version for exn to 0.3.1.
CHANGELOG.md	Adds release notes for v0.3.1 dated 2026-05-06.

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