Add CLAUDE.md project documentation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+OpenNest is a Windows desktop application for CNC nesting — arranging 2D parts on material plates to minimize waste. It imports DXF drawings, places parts onto plates using rectangle-packing algorithms, and can export nest layouts as DXF or post-process them to G-code for CNC cutting machines.
+
+## Build
+
+This is a .NET Framework 4.8 solution using legacy-style `.csproj` files (not SDK-style). Build with:
+
+```bash
+msbuild OpenNest.sln /p:Configuration=Release
+```
+
+NuGet dependency: `netDxf` (referenced via `packages/` folder, not PackageReference). Restore with `nuget restore OpenNest.sln` if needed.
+
+No test projects exist in this solution.
+
+## Architecture
+
+Three projects form a layered architecture:
+
+### OpenNest.Core (class library)
+Domain model and geometry primitives. Everything lives in the `OpenNest` namespace.
+
+- **Domain model**: `Nest` → `Plate[]` → `Part[]` → `Drawing` → `Program`. A `Nest` is the top-level container. Each `Plate` has a size, material, quadrant, spacing, and contains placed `Part` instances. Each `Part` references a `Drawing` (the template) and has its own location/rotation. A `Drawing` wraps a CNC `Program`.
+- **CNC subsystem** (`CNC/`): `Program` holds a list of `ICode` instructions (G-code-like: `RapidMove`, `LinearMove`, `ArcMove`, `SubProgramCall`). Programs support absolute/incremental mode conversion, rotation, offset, bounding box calculation, and cloning. This is the core geometric representation — parts are manipulated by transforming their `Program`.
+- **Geometry** (`Geometry/`): Higher-level shapes (`Line`, `Arc`, `Circle`, `Polygon`, `Shape`) used for intersection detection, area calculation, and DXF conversion. `ConvertProgram.ToGeometry()` bridges from CNC codes to geometry entities.
+- **Quadrant system**: Plates use quadrants 1-4 (like Cartesian quadrants) to determine coordinate origin placement. This affects bounding box calculation, rotation, and part positioning.
+
+### OpenNest.Engine (class library, depends on Core)
+Nesting algorithms. `NestEngine` orchestrates filling plates with parts.
+
+- **RectanglePacking/**: `FillBestFit` (single-item fill, tries horizontal and vertical orientations), `PackBottomLeft` (multi-item bin packing, sorts by area descending). Both operate on `Bin`/`Item` abstractions.
+- **CirclePacking/**: Alternative packing for circular parts.
+- `NestItem`: Input to the engine — wraps a `Drawing` with quantity, priority, and rotation constraints.
+- `BestCombination`: Finds optimal mix of normal/rotated columns for grid fills.
+
+### OpenNest (WinForms WinExe, depends on Core + Engine)
+The UI application with MDI interface.
+
+- **Forms/**: `MainForm` (MDI parent), `EditNestForm` (MDI child per nest), plus dialogs for plate editing, auto-nesting, DXF conversion, cut parameters, etc.
+- **Controls/**: `PlateView` (2D plate renderer with zoom/pan), `DrawingListBox`, `DrawControl`, `QuadrantSelect`.
+- **Actions/**: User interaction modes — `ActionSelect`, `ActionAddPart`, `ActionClone`, `ActionFillArea`, `ActionZoomWindow`, `ActionSetSequence`.
+- **IO/**: `DxfImporter`/`DxfExporter` (via netDxf library), `NestReader`/`NestWriter` (custom ZIP-based format with XML metadata + G-code programs), `ProgramReader`.
+- **Post-processing**: `IPostProcessor` plugin interface loaded from DLLs in a `Posts/` directory at runtime.
+
+## File Format
+
+Nest files (`.zip`) contain:
+- `info` — XML with nest metadata and plate defaults
+- `drawing-info` — XML with drawing metadata (name, material, quantities, colors)
+- `plate-info` — XML with plate metadata (size, material, spacing)
+- `program-NNN` — G-code text for each drawing's cut program
+- `plate-NNN` — G-code text encoding part placements (G00 for position, G65 for sub-program call with rotation)
+
+## Key Patterns
+
+- All namespace roots are `OpenNest` (even OpenNest.Core uses `namespace OpenNest`).
+- `ObservableList<T>` provides ItemAdded/ItemRemoved/ItemChanged events used for automatic quantity tracking between plates and drawings.
+- Angles throughout the codebase are in **radians** (use `Angle.ToRadians()`/`Angle.ToDegrees()` for conversion).
+- `Tolerance.Epsilon` is used for floating-point comparisons across geometry operations.