feat: improve documentation quality with real data

- Extract file-level docstrings from Python files (module-level string expressions)
- Use __init__.py docstrings as module doc_summary
- Use file docstrings as file purpose in layout tables (instead of 'Source file')
- Populate module outbound_modules/inbound_modules from import edges (internal only)
- Make filename sanitization consistent (sanitize_for_link matches sanitize_filename)
- Clean up stale .md files from previous runs before generating
- Fill ARCHITECTURE.md template with real layout, modules index, and critical points
- Add file_docstring field to ParsedModule and file_purpose to FileDoc

PR_DESCRIPTION.md

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+# PR: Major improvements to ArchDoc
+
+## Summary
+
+Comprehensive refactoring and feature additions to ArchDoc — the Python architecture documentation generator. This PR improves code quality, adds new features, and significantly enhances the development experience.
+
+**Stats:** 24 files changed, ~3900 insertions, ~1400 deletions, 50 tests
+
+## Changes
+
+### 🏗️ Architecture
+- **Decomposed monolithic `main.rs`** into `commands/` module structure (generate, init, check, stats)
+- **Added workspace `Cargo.toml`** for unified builds across both crates
+- **New `cycle_detector` module** with DFS-based dependency cycle detection
+
+### 🐍 Python Analyzer
+- **Full AST traversal** — properly walks all statement types (if/for/while/try/with/match)
+- **Function signatures** — extracts parameter names, types, defaults, return types
+- **Method detection** — distinguishes methods from standalone functions via `self`/`cls` parameter
+- **Docstring extraction** — parses first line of docstrings for symbol documentation
+- **Module path computation** — correctly computes module IDs from `src_roots` config
+
+### ✨ New Features
+- **`stats` command** — project statistics with colored output and progress bar
+- **Config validation** — validates project root, language, scan paths, cache age, file size formats
+- **Cycle detection** — finds circular dependencies in module graph, shown in critical points section
+- **`--dry-run` flag** — preview what would be generated without writing files
+- **Dynamic project data** — uses config project name and current date instead of hardcoded values
+- **Real usage examples** — generates Python import/call examples from analyzed symbols
+- **Skip-unchanged optimization** — writer skips files that haven't changed
+
+### 🧹 Code Quality
+- **Zero `unwrap()` calls** in non-test code — proper error handling throughout
+- **Zero clippy warnings** — all lints resolved
+- **50 tests** — unit tests for config validation, cycle detection, caching, integration detection, error handling, and full pipeline integration tests
+
+### 📚 Documentation
+- **README.md** — badges, full command reference, configuration table, architecture overview
+- **CHANGELOG.md** — complete changelog for this branch
+
+## Testing
+
+```bash
+cargo test        # 50 tests, all passing
+cargo clippy      # 0 warnings
+cargo build       # clean build
+```
+
+## Breaking Changes
+
+None. All existing functionality preserved.