Add CLAUDE.md for Claude Code integration (#1621)

## Summary
- Added CLAUDE.md file to provide guidance for Claude Code when working
with the UnitsNet codebase
- Includes essential commands, architecture overview, and development
guidelines
- Helps Claude Code understand the unique code generation workflow

## Test plan
- [x] Created CLAUDE.md file with comprehensive documentation
- [x] Verified file includes all key commands and workflows
- [x] Documented code generation process and project structure

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <[email protected]>

Author: angularsen

CLAUDE.md

@@ -0,0 +1,119 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+UnitsNet is a .NET library that provides strongly-typed physical units and quantities, enabling safe and intuitive unit conversions in code. The library uses code generation from JSON definitions to create type-safe APIs for over 130 physical quantities.
+
+## Key Commands
+
+### Build and Test
+- **Build project**: `build.bat` or `dotnet build UnitsNet.slnx`
+- **Build all targets** (including nanoFramework): `build-all-targets.bat`
+- **Run tests**: `test.bat` or `dotnet test UnitsNet.slnx`
+- **Run single test**: `dotnet test UnitsNet.Tests --filter "FullyQualifiedName~TestClassName.TestMethodName"`
+- **Clean artifacts**: `clean.bat`
+
+### Code Generation
+- **Generate code from JSON definitions**: `generate-code.bat` or `dotnet run --project CodeGen`
+  - Always run this after modifying any JSON files in `Common/UnitDefinitions/`
+  - The generator reads 131 JSON definition files and creates C# code
+
+### Development Workflow
+1. Modify unit definitions in `Common/UnitDefinitions/*.json`
+2. Run `generate-code.bat` to regenerate C# code
+3. Run `build.bat` to compile and test
+4. Use `test.bat` for isolated test runs
+
+## Code Architecture
+
+### Project Structure
+- **UnitsNet/**: Main library with quantity types and units
+  - `GeneratedCode/`: Auto-generated from JSON definitions (do not edit manually)
+  - `CustomCode/`: Hand-written code extending generated types
+- **UnitsNet.Tests/**: Comprehensive test suite
+- **CodeGen/**: Code generation tool that creates C# from JSON definitions
+- **Common/UnitDefinitions/**: 131 JSON files defining physical quantities
+- **UnitsNet.NumberExtensions/**: Extension methods for numeric types
+- **UnitsNet.Serialization.*/**: JSON.NET and System.Text.Json serialization support
+- **UnitsNet.NanoFramework/**: Support for embedded .NET nanoFramework
+
+### Code Generation Process
+The project uses a sophisticated code generation system:
+1. JSON definitions in `Common/UnitDefinitions/` describe units, conversions, and localizations
+2. `CodeGen` project processes these to generate:
+   - Quantity types (e.g., `Length`, `Mass`)
+   - Unit enums (e.g., `LengthUnit`, `MassUnit`)
+   - Conversion logic and unit abbreviations
+3. Generated code goes to `*/GeneratedCode/` folders
+4. Custom code in `*/CustomCode/` extends generated types
+
+### Key Classes and Patterns
+- **IQuantity**: Base interface for all quantity types
+- **Quantity**: Static class for dynamic quantity operations
+- **UnitConverter**: Handles conversions between units
+- **QuantityParser/UnitParser**: Parse strings to quantities/units
+- **UnitsNetSetup**: Configuration singleton
+
+### Adding or Modifying Units
+1. Edit or create JSON file in `Common/UnitDefinitions/`
+2. Follow conversion function guidelines:
+   - Use multiplication for `FromUnitToBaseFunc`
+   - Use division for `FromBaseToUnitFunc`
+   - Prefer scientific notation (1e3, 1e-5)
+3. Run `generate-code.bat`
+4. Add tests if needed
+
+## Important Conventions
+
+### Coding Standards
+- Follow `.editorconfig` specifications
+- Use ReSharper settings in `UnitsNet.sln.DotSettings`
+- Treat warnings as errors (except obsolete warnings)
+- Add file headers to new files
+
+### Unit Definition Rules
+- Base units are chosen for each quantity (e.g., meter for Length)
+- All conversions go through the base unit
+- Use superscript in abbreviations: cm², m³
+- Compound units format: N·m (dot), km/h (slash)
+
+### Testing
+- Test class naming: `<Type>Tests`
+- Test method naming: `<method>_<condition>_<result>`
+- Tests accept error margin of 1E-5 for most units
+
+## Special Considerations
+
+### .NET nanoFramework Support
+- Separate projects for nanoFramework compatibility
+- Use `build-all-targets.bat` to include nanoFramework builds
+- Limited feature set compared to full .NET
+
+### Performance
+- Conversion functions are compiled to delegates for performance
+- All conversions go through base units (potential for small errors)
+- Precision goal is 1E-5 for most units
+
+### Localization
+- Unit abbreviations support multiple cultures
+- JSON definitions include translations for various languages
+- Default culture: Thread.CurrentCulture, fallback to en-US
+
+## Common Tasks
+
+### Find specific quantity or unit implementation
+- Quantity types: `UnitsNet/GeneratedCode/Quantities/*.g.cs`
+- Unit enums: `UnitsNet/GeneratedCode/Units/*.g.cs`
+- Custom extensions: `UnitsNet/CustomCode/Quantities/*.extra.cs`
+- Unit definitions: `Common/UnitDefinitions/*.json`
+
+### Debug code generation
+- Generator entry: `CodeGen/Program.cs`
+- Generator logic: `CodeGen/Generators/`
+- Enable verbose logging: Check Serilog configuration in Program.cs
+
+### Run performance benchmarks
+- Execute: `dotnet run -c Release --project UnitsNet.Benchmark`
+- Results saved to `Artifacts/` folder