chore(release): sync versions to 0.9.420 [skip release]

[github-actions]

Automated write-back from auto-tag.yml: v0.9.420 was just released, and this PR records that version everywhere the sources carry one —

• [workspace.package].version in Cargo.toml (inherited by every crate)
• the workspace-member entries in Cargo.lock
• packaging/homebrew/stella.rb (build-from-source formula)
• CHANGELOG.md — whatever was under [Unreleased] now sits under [0.9.420]

The workflow approves this PR's parked workflow runs, waits for the required checks, and merges once they are green (#5589). No action is needed unless a check fails.

Summary by Sourcery

Synchronize project version metadata and release notes for version 0.9.420.

Build:

• Update the workspace and Homebrew package metadata to version 0.9.420.

Documentation:

• Move the changelog entries from Unreleased into the 0.9.420 release section.

Chores:

• Synchronize workspace lockfile package versions with the 0.9.420 release.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

This automated release write-back records version 0.9.420 consistently in workspace metadata, lockfile package entries, the Homebrew build-from-source formula, and the changelog.

File-Level Changes

Change	Details	Files
Synchronize the repository’s release version to 0.9.420 across package metadata and lockfile records.	Update the workspace-inherited package version from 0.9.419 to 0.9.420. Refresh workspace-member versions in the lockfile.	Cargo.toml Cargo.lock
Update the Homebrew source formula to build the newly released tag.	Point the formula URL tag and displayed version at v0.9.420. Leave the existing build dependency and formula configuration unchanged.	packaging/homebrew/stella.rb
Finalize the changelog section for the new release.	Move entries previously under [Unreleased] into [0.9.420].	CHANGELOG.md

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[sourcery-ai]

Hey - I've reviewed your changes and they look great!

Sourcery assessment

Approved.

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