chore(release): prepare changelog for v0.16.9

[KaiPilotBot]

Automated by the Release — Prepare Changelog workflow.

Folds the pending .changes/unreleased/ fragments into CHANGELOG.md as ## [v0.16.9] and clears them. CHANGELOG.md is the source of truth.

Merging this PR triggers release-publish.yaml, which tags v0.16.9 and creates the GitHub Release from this changelog section.

• Review the new ## [v0.16.9] section for accuracy.
• Target branch: v0.16.

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