docs: add CHANGELOG entry for session 9 benchmark additions

Author: hyperpolymath

CHANGELOG.md

@@ -5,6 +5,11 @@ All notable changes to iseriser 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]
+
+### Added (2026-04-04)
+- Criterion benchmark suite (`benches/iseriser_bench.rs`) — 8 benchmarks covering codegen performance and manifest parsing efficiency
+
 ## [0.1.0] - 2026-03-20
 
 ### Added