feat: Complete Sprint 6 - Testing & Documentation

Sprint 6A: Edge Case Tests
- SyntaxErrorTests for parse error handling
- EncodingTests for UTF-8/UTF-16 and Unicode
- WhitespaceTests for whitespace-only changes
- ModernCSharpTests (records, primary constructors, raw strings, etc.)
- PreprocessorTests for #if/#endif directives
- GenericTests for generic classes/methods
- 160 new edge case tests

Sprint 6B: Integration Tests
- New RoslynDiff.Integration.Tests project
- End-to-end C#, VB.NET, and line diff tests
- ClassCommand integration tests
- Output format integration tests
- Error handling tests
- 115 integration tests

Sprint 6C: Documentation
- Complete README.md with installation and usage
- docs/usage.md - Detailed CLI documentation
- docs/output-formats.md - Format descriptions
- docs/api.md - Programmatic API reference
- docs/architecture.md - System design overview
- docs/performance.md - Performance characteristics
- CHANGELOG.md - Version history
- samples/ - Example files and outputs

Sprint 6D: Performance Tests
- RoslynDiff.Benchmarks project with BenchmarkDotNet
- CSharpDifferBenchmarks for diff performance
- SyntaxComparerBenchmarks for comparison speed
- OutputFormatterBenchmarks for formatting
- PerformanceTests.cs for timeout validation

Test Results: 650 tests passing
- Core: 321 tests (+160)
- Output: 130 tests
- CLI: 84 tests
- Integration: 115 tests (new)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: randlee

CHANGELOG.md

@@ -0,0 +1,130 @@
+# 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).
+
+## [Unreleased]
+
+### Planned
+- MCP Server integration for AI tooling
+- Folder/project comparison
+- Git integration (compare commits, branches)
+- F# support via FSharp.Compiler.Service
+
+## [1.0.0] - 2025-XX-XX
+
+### Added
+
+#### Core Features
+- **C# Semantic Diff** - Full semantic analysis of C# source files using Roslyn
+- **VB.NET Semantic Diff** - Full semantic analysis of Visual Basic source files
+- **Line-by-Line Diff** - Fallback diff for non-.NET files using DiffPlex
+- **Rename Detection** - Detects when symbols are renamed (not just added/removed)
+- **Move Detection** - Identifies code that has been moved within a file
+
+#### Change Detection
+- Class/struct/record/interface changes
+- Method/function changes
+- Property changes
+- Field changes
+- Namespace changes
+- Statement-level changes (in Roslyn mode)
+
+#### CLI Commands
+- `diff` - Compare two files
+  - `--mode` option: `auto`, `roslyn`, `line`
+  - `--ignore-whitespace` / `-w` - Ignore whitespace differences
+  - `--ignore-comments` / `-c` - Ignore comment differences
+  - `--context` / `-C` - Control context lines
+  - `--output` / `-o` - Select output format
+  - `--out-file` - Write to file
+  - `--rich` / `-r` - Rich terminal output
+
+- `class` - Compare specific classes between files
+  - `--match-by` / `-m` - Match strategy: `exact`, `interface`, `similarity`, `auto`
+  - `--interface` / `-i` - Interface name for interface matching
+  - `--similarity` / `-s` - Similarity threshold
+  - `--output` / `-o` - Select output format
+  - `--out-file` / `-f` - Write to file
+
+#### Output Formats
+- **JSON** - Machine-readable format for AI/tooling
+- **HTML** - Interactive report with syntax highlighting
+- **Text** - Unified diff format (similar to git diff)
+- **Plain** - Simple text without ANSI codes
+- **Terminal** - Rich console output with Spectre.Console
+
+#### Class Matching Strategies
+- Exact name matching
+- Interface implementation matching
+- Content similarity matching (for renamed classes)
+- Auto mode (exact first, then similarity)
+
+#### API
+- `DifferFactory` for creating appropriate differs
+- `IDiffer` interface for custom differ implementations
+- `OutputFormatterFactory` for creating formatters
+- `IOutputFormatter` interface for custom formatters
+- `ClassMatcher` for flexible class matching
+
+### Dependencies
+- .NET 10.0
+- Microsoft.CodeAnalysis.CSharp 4.12.0
+- Microsoft.CodeAnalysis.VisualBasic 4.12.0
+- DiffPlex 1.7.2
+- Spectre.Console 0.49.x
+- System.CommandLine 2.0.0-beta4
+
+---
+
+## Version History
+
+### Versioning Strategy
+
+This project follows [Semantic Versioning](https://semver.org/):
+
+- **MAJOR** version for incompatible API changes
+- **MINOR** version for backward-compatible functionality additions
+- **PATCH** version for backward-compatible bug fixes
+
+### Release Phases
+
+| Phase | Version | Features |
+|-------|---------|----------|
+| Phase 1 | v1.0.0 | Core CLI (current) |
+| Phase 2 | v1.x | MCP Server integration |
+| Phase 3 | v2.0 | Folder/project/git support |
+| Phase 4 | v3.0 | Solution-level comparison |
+
+---
+
+## Migration Guide
+
+### From 0.x to 1.0
+
+If upgrading from pre-release versions:
+
+1. **CLI Changes**
+   - `--format` renamed to `--output` / `-o`
+   - Default output format is now `text` (was `terminal`)
+   - Use `--rich` / `-r` for colored terminal output
+
+2. **API Changes**
+   - `DiffResult` is now a record type (immutable)
+   - `Change.Location` split into `OldLocation` and `NewLocation`
+   - `OutputFormatter` interface renamed to `IOutputFormatter`
+
+3. **JSON Output Schema**
+   - Schema version updated to `roslyn-diff-output-v1`
+   - `summary` object structure changed
+   - Location objects now include `startColumn` and `endColumn`
+
+---
+
+## Links
+
+- [GitHub Repository](https://github.com/randlee/roslyn-diff)
+- [Issue Tracker](https://github.com/randlee/roslyn-diff/issues)
+- [Documentation](./docs/)