docs: add CHANGELOG.md starting at v0.5.0

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+## [0.5.0]
+
+### Added
+
+- `RenameFS` interface and top-level `wfs.Rename` helper for atomic
+  cross-filesystem rename support (#6).
+- `SyncWriterFile` capability interface so callers can fsync a
+  `WriterFile` before closing it. `osfs` wraps `(*os.File).Sync` and
+  `memfs` implements it as a no-op so the same caller code works on
+  both backends (#6).
+- `wfstest.TestRenameFS` reusable conformance test for `RenameFS`
+  implementations (#6).
+- CI: `staticcheck` and `gosec` are now run on every push and pull
+  request (#7).
+- CI: Go version matrix (`1.24`, `1.25`, `stable`) so regressions
+  against the lowest supported toolchain are caught.
+
+### Changed
+
+- Minimum Go version is now 1.24 (was 1.26) so projects on older
+  toolchains can consume wfs.
+- `memfs` now documents in the `MemFile` godoc that writes are buffered
+  locally and only published on `Close`, that `Sync` is a no-op, and
+  that the same applies when used with the new `SyncWriterFile`
+  capability (#10).
+
+### Fixed
+
+- `memfs.MemFS.Sub` previously returned a new `MemFS` with a fresh
+  `sync.Mutex` while sharing the underlying store. Concurrent access
+  through a parent and any of its sub-filesystems therefore raced.
+  `Sub` now shares the parent's mutex pointer so all views of the same
+  store serialize through a single lock (#8).
+
+### Performance
+
+- `memfs/store.put` no longer calls `sort.Strings` on every insert.
+  Bulk loads are now O(n) per put instead of O(n log n), giving roughly
+  a 30x speedup on the included `BenchmarkStore_put` (#9).
+- `memfs/store.removeAll` uses binary search to find the end of the
+  prefix range instead of a linear scan (#9).
+
+## [0.4.1] and earlier
+
+See the git log.
+
+[Unreleased]: https://github.com/mojatter/wfs/compare/v0.5.0...HEAD
+[0.5.0]: https://github.com/mojatter/wfs/compare/v0.4.1...v0.5.0