Fix triple `[Unreleased]` heading merge artifact in CHANGELOG.md

[PrincessnJoy]

Description

CHANGELOG.md currently contains three separate ## [Unreleased] headings — a clear merge conflict artifact where the same section was triplicated instead of being properly resolved.

This breaks:

• Automated changelog parsing tools (e.g., git-cliff, standard-version, release scripts) that expect exactly one [Unreleased] section
• The release.yml GitHub Actions workflow, which likely extracts the [Unreleased] block to generate release notes — it will either pick the first (possibly empty) block or concatenate all three
• Manual readability — contributors scanning the changelog to understand what's queued for the next release will see confusing duplicates

The only released version currently documented is ## [1.0.0] — 2026-05-17.

Steps to Reproduce

grep -n '\[Unreleased\]' CHANGELOG.md

Should output exactly one line. Currently outputs three.

Acceptance Criteria

• Merge all three ## [Unreleased] sections into a single, coherent ## [Unreleased] section at the top of the file
• The merged section contains all unique entries from the three duplicate blocks (no entries are lost)
• The ## [1.0.0] section and all subsequent released version sections are left unchanged
• grep -c '\[Unreleased\]' CHANGELOG.md outputs 1
• The CHANGELOG follows the Keep a Changelog format (Added / Changed / Deprecated / Removed / Fixed / Security subsections)
• If a changelog linter is available in CI (e.g., git-cliff --check), it passes cleanly

Priority

Low — does not affect runtime behavior, but causes release automation failures.

Estimated Effort

Small