Prepare release 0.9.0

[hexsecs]

Goal

Prepare CANarchy release 0.9.0 from the current main branch.

Note on version target

main is currently on 0.8.1.dev0 (the default next-patch target after 0.8.0). This release intentionally targets 0.9.0 (a minor bump) instead, per the documented override in docs/release.md ("if the next planned release target is a minor or major bump instead of the next patch version, document that decision explicitly").

Acceptance Criteria

• Version metadata is set to 0.9.0.
• CHANGELOG.md promotes [Unreleased] entries to a 0.9.0 release heading with the correct date.
• Release prep verification passes: tests, docs build, package build, and relevant smoke checks.
• A release-prep PR is opened for review.

Notes

This issue covers release preparation only. Tagging/publishing happens after the PR is reviewed and merged.