chore(changelog): keep the B-tree entry under [Unreleased] after the v2.4.0 cut

v2.4.0 was tagged on main while this branch was open, which inserted a
"## [2.4.0]" heading directly under "## [Unreleased]". The rebase carried this
PR's entry into the released section; it belongs above it, under a fresh
[Unreleased] -> Added.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: marius-bughiu

CHANGELOG.md

@@ -4,11 +4,14 @@ All notable changes to Celerity are documented here. This project follows [Keep
 
 ## [Unreleased]
 
+### Added
+
+- **`BTreeDictionary<TKey, TValue, TComparer>` and `BTreeSet<T, TComparer>`** (with `BTreeDictionary<TKey, TValue>` / `BTreeSet<T>` aliases and the `DefaultComparer<T>` struct comparer) in `Celerity.Collections` — the library's first sorted map and set, and the B-tree the BCL lacks. Up to 31 keys per node keep a lookup `log₃₂(n)` node visits deep instead of chasing the `log₂(n)` pointers a red-black tree costs, and both add the ordered surface a hash table cannot answer: `Min` / `Max`, lower / upper bound, `EnumerateRange` in `O(log n + k)`, and in-order enumeration. They win on the interleaved insert + lookup + range-scan workload and on memory, and lose slightly on a delete-dominated one. Not thread-safe. Closes [#305](https://github.com/marius-bughiu/Celerity/issues/305).
+
 ## [2.4.0] - 2026-07-26
 
 ### Added
 
-- **`BTreeDictionary<TKey, TValue, TComparer>` and `BTreeSet<T, TComparer>`** (with `BTreeDictionary<TKey, TValue>` / `BTreeSet<T>` aliases and the `DefaultComparer<T>` struct comparer) in `Celerity.Collections` — the library's first sorted map and set, and the B-tree the BCL lacks. Up to 31 keys per node keep a lookup `log₃₂(n)` node visits deep instead of chasing the `log₂(n)` pointers a red-black tree costs, and both add the ordered surface a hash table cannot answer: `Min` / `Max`, lower / upper bound, `EnumerateRange` in `O(log n + k)`, and in-order enumeration. They win on the interleaved insert + lookup + range-scan workload and on memory, and lose slightly on a delete-dominated one. Not thread-safe. Closes [#305](https://github.com/marius-bughiu/Celerity/issues/305).
 - **Native AOT smoke-test coverage for `Trie<TValue>`** — the trie was the one collection the AOT smoke test did not exercise, so its surface is now proven to run under a Native AOT publish. Test-only hardening; no public API change.
 - **`FenwickTree<T>`** in `Celerity.Collections` — a Binary Indexed Tree over a fixed-length numeric sequence (`where T : struct, INumber<T>`) that applies point updates and answers prefix / range sums both in `O(log n)`, filling a BCL gap: .NET ships no prefix-sum structure, and a plain array costs `O(n)` per query or `O(n)` per update. It wins precisely where updates and range-sum queries interleave — running aggregates, rank / order-statistics counters, cumulative-frequency tables. Not thread-safe. Closes [#289](https://github.com/marius-bughiu/Celerity/issues/289).
 - **`Trie<TValue>`** in `Celerity.Collections` — an ordered prefix tree mapping `string` keys to values, filling a BCL gap (.NET ships no trie). `GetByPrefix` lists every entry whose key starts with a prefix in `O(prefix + matches)` and in ascending key order, and `TryGetLongestPrefix` finds the longest stored key that is a prefix of a query in `O(query)` — the autocomplete, longest-prefix-routing, and ordered-iteration workloads a `Dictionary<string, TValue>` can only answer with an `O(n)` scan plus a `StartsWith` per key. Exact `Add` / `TryGetValue` favour a `Dictionary`, so the trie earns its place on the prefix operations. Implements `IReadOnlyDictionary<string, TValue?>`; not thread-safe. Closes [#285](https://github.com/marius-bughiu/Celerity/issues/285).