chore: add initial CHANGELOG

Author: schroedk

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).
+
+## [0.1.0] - 2026-03-10
+
+Initial release of `approx-chol`, providing approximate Cholesky factorization
+for graph Laplacians in Rust with Python bindings.
+
+### Added
+
+- Core approximate Cholesky factorization algorithm for SDD/SDDM matrices
+- Fallible APIs with structured error types throughout
+- CSR sparse matrix representation with checked accessors
+- Triangular solve (`solve` and `solve_into`) for factored systems
+- Clique sampling and star/clique-tree internals
+- Serialization support via optional `serde` feature
+- Python bindings (via PyO3/maturin) exposing factorization and solve
+- Input validation and borrow-safe `solve_into` on the Python side
+- Proptest suites for CSR construction and factorization invariants
+- Preconditioner effectiveness tests
+- Criterion benchmarks for factorization, sampling, solve, and CSR conversion
+- Multi-platform CI with coverage reporting
+- Dual MIT license for Rust crate and Python package
+
+[0.1.0]: https://github.com/aai-institute/approx-chol/releases/tag/v0.1.0