docs(changelog): open 0.5.0 section

Skeleton Added/Changed/Fixed/Removed subsections + compare-link pointing
at main for the next development cycle.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: dfa1

CHANGELOG.md

@@ -5,6 +5,18 @@ All notable changes to **vortex-java** are documented here.
 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).
 
+## [0.5.0] — Unreleased
+
+### Added
+
+### Changed
+
+### Fixed
+
+### Removed
+
+[0.5.0]: https://github.com/dfa1/vortex-java/compare/v0.4.0...main
+
 ## [0.4.0] — 2026-06-07
 
 The headline themes for this release are a **security-hardening sweep** of the file-format