Byte alignment and arrow feature

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 and this project adheres to
+(or is loosely based on) Semantic Versioning.
+
+## [Unreleased]
+
+### Added
+### Changed
+### Deprecated
+### Removed
+### Fixed
+### Security
+### Migration
+- If there are breaking changes, put a short, actionable checklist here.
+
+## [0.14.0-alpha] - 2024-09-08
+### Breaking
+- Files written by 0.14.0-alpha use padded payload starts for fixed alignment.
+  Older readers (<= 0.13.x-alpha) may misinterpret pre-pad bytes as part of the
+  payload. Upgrade all readers/writers before mixing file versions.
+
+### Added
+- Fixed payload alignment for zero-copy typed views. Payloads now begin
+  at an address that is a multiple of `PAYLOAD_ALIGNMENT`, configured in
+  `src/storage_engine/constants.rs` via:
+  - `PAYLOAD_ALIGN_LOG2`
+  - `PAYLOAD_ALIGNMENT = 1 << PAYLOAD_ALIGN_LOG2`
+- Experimental `arrow` feature which exposes `as_arrow_buffer` and `into_arrow_buffer`
+  methods in `EntryHandle`.
+
+### Changed
+- Internal on-disk layout: each non-tombstone payload may be preceded by
+  a small zero pre-pad (0..A-1 bytes) to satisfy alignment (A is the
+  configured alignment). Public API is unchanged.
+
+### Migration
+- Regenerate stores with the new version:
+  1) Open the old store with the matching old binary and read entries.
+  2) Write each entry into a new 0.14.0-alpha store.
+  3) Replace the old file after verification.
+- If you maintain separate services, deploy reader upgrades before
+  writer upgrades to avoid mixed-version reads.