docs: tighten unreleased notes

[Chemaclass]

🤔 Background

Make the Unreleased notes easier to review before choosing the next release.

💡 Changes

• Put runner and reporting improvements before contributor and CI changes.
• Tighten wording while preserving all entries, references and past releases.
• Correct the metadata cache limit from 512 characters to 512 bytes.
• Validate sequential and parallel suites, static analysis and formatting.