docs: add CHANGELOG.md documenting all branch changes

CHANGELOG.md

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+# Changelog
+
+All notable changes to ArchDoc are documented in this file.
+
+Format follows [Keep a Changelog](https://keepachangelog.com/).
+
+## [Unreleased] — feature/improvements-v2
+
+### Added
+- **Config validation** (`Config::validate()`) — checks project root, language, scan includes, src_roots, cache age, and file size formats with helpful error messages
+- **Duration & file size parsers** — `parse_duration()` (s/m/h/d/w) and `parse_file_size()` (B/KB/MB/GB) utility functions
+- **Dependency cycle detection** (`cycle_detector.rs`) — DFS-based algorithm to find circular module dependencies
+- **Cycle detection in renderer** — Critical points section now shows detected dependency cycles
+- **Full pipeline integration tests** — Tests for config validation, scanning, cycle detection, and rendering
+- **Stats command** — `archdoc stats` displays project-level statistics (files, modules, symbols, edges)
+- **Check command** — `archdoc check` verifies documentation consistency with code
+- **Colored CLI output** — Progress bars and colored status messages
+- **Comprehensive README** — Badges, configuration reference table, command documentation, architecture overview
+
+### Changed
+- **CLI architecture** — Decomposed into separate command modules (generate, check, stats, init)
+- **Error handling** — Improved error messages with `thiserror` and `anyhow`
+- **Clippy compliance** — All warnings resolved
+
+### Fixed
+- Various clippy warnings and code style issues