docs(changelog): add [2.2.0] section; collapse duplicate [Unreleased] Added (#251)

[marius-bughiu]

Fixes #251.

Problem

v2.2.0 was tagged and released (2026-07-05), but CHANGELOG.md had no ## [2.2.0] section — the heading sequence jumped straight from ## [Unreleased] to ## [2.1.0]. Everything that shipped in v2.2.0 was stranded under [Unreleased], mixed with genuinely-unreleased post-2.2.0 work, and [Unreleased] carried two ### Added subsections (a Keep-a-Changelog hygiene violation and the visible symptom of the release rollover never happening).

Because release.yml extracts GitHub Release notes by the ## [X.Y.Z] heading, v2.2.0's release notes would have come out empty.

Fix

Split at the real release boundary, verified via git log v2.1.0..v2.2.0 (shipped) and git log v2.2.0..HEAD (unreleased):

• New ## [2.2.0] - 2026-07-05 (tag date) with a single ### Added: CelerityMultiSet (CelerityMultiSet<T, THasher> — counting multiset (bag/counter), the MultiMap sibling #235), SwissSet, TopKSketch (TopKSketch<T, THasher> — Space-Saving top-k / heavy-hitters sketch #238), and the ISet<T> set-algebra surface (Set-algebra parity: implement ISet<T> on the mutable set family (CeleritySet / IntSet / LongSet / SwissSet) #240).
• [Unreleased] collapsed to a single ### Added. BitWriter/BitReader (feat(primitives): add BitWriter/BitReader — sequential sub-byte bit I/O #242) — which landed after the v2.2.0 tag — stays here alongside RobinHoodSet, HashCachingSet, PooledCeleritySet, and all newer work. Note the subtlety the issue flagged: BitWriter/BitReader sat in the second ### Added block next to the v2.2.0 items but belongs in [Unreleased].

Net diff: +4 / −2 lines — two heading changes only, no changelog prose touched.

release.yml sanity-check (issue step 3)

No change needed. The extraction step already keys off the ## [<version>] heading exactly and fails loudly on an empty section:

index($0, "## [" ver "]") == 1 { capture = 1; next }
capture && index($0, "## [") == 1 { exit }
capture { print }

The root cause was purely the missing heading. Verified locally that the awk now emits exactly the four v2.2.0 entries for [2.2.0] and stops cleanly at [2.1.0], with [2.1.0] extraction unaffected.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR updates the project changelog to properly reflect the already-released v2.2.0 by introducing a missing ## [2.2.0] - 2026-07-05 section and eliminating the duplicate ### Added subsection under [Unreleased], ensuring release-note extraction and Keep-a-Changelog structure behave correctly.

Changes:

• Add the missing ## [2.2.0] - 2026-07-05 section with a single ### Added header to capture the shipped v2.2.0 entries.
• Collapse [Unreleased] to a single ### Added subsection by removing the duplicate heading.