docs(changelog): add 0.2.1 entry for evaluate_batch hook

[CooperBigFoot]

What

Adds a [0.2.1] section to CHANGELOG.md (dated 2026-06-29) documenting the optional
evaluate_batch hook on ga() / nsga2() (whole-population batched fitness/objective
evaluation, shipped in commit 5a69300), and fixes the changelog link references:
[Unreleased] now compares from v0.2.1, and a new [0.2.1] release-tag link is added.

Scope

• Only CHANGELOG.md is changed. No source, tests, workflows, or version bumps —
  pyproject.toml is already at 0.2.1 and __version__ is dynamic.

Release note (human action required after merge)

This PR does not tag or release anything. After it merges, a maintainer should tag
v0.2.1 and cut a non-prerelease GitHub Release, which triggers release.yml to
publish ctrl-freak 0.2.1 to PyPI via OIDC Trusted Publishing.

[CooperBigFoot]

Automated PR review (fresh adversarial reviewer) — APPROVE

Verified against step-0-ctrlfreak-release/plan.md and MILESTONE §2/§4:

• Scope: Only CHANGELOG.md changed (12 +, 1 -). No pyproject.toml, __init__.py, workflows, source, or tests touched. Base main, head step/ctrlfreak-0.2.1-changelog. ✓
• Section: ## [0.2.1] - 2026-06-29 inserted between [Unreleased] and [0.2.0] with ### Added. [Unreleased] left as a bare heading. ✓
• Accuracy (cross-checked ga.py / nsga2.py): The bullet mirrors the docstrings — whole (pop_size, n_params) matrix in one call, bypasses the per-individual evaluate/lift loop, and falls back byte-for-byte identical when omitted. Both ga() and nsga2() carry the evaluate_batch=None param (ga.py:50, nsga2.py:50). Not overclaiming. ✓
• Links: [Unreleased] → compare/v0.2.1...HEAD; new [0.2.1] uses releases/tag/v0.2.1 (tag form, matching [0.2.0]/[0.1.0], not compare form); siblings unchanged; order [Unreleased], [0.2.1], [0.2.0], [0.1.0]. ✓
• Human gates intact: no v0.2.1 git tag, no GitHub Release, no PyPI publish in this PR. ✓
• Formatting: Keep-a-Changelog style consistent with the rest of the file; markdown well-formed. ✓

No defects found. Ready to merge (G1); tagging/release remain separate human gates (G2).