docs: add GPU bitmap best fit evaluation design

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

docs/plans/2026-03-07-gpu-bitmap-bestfit-design.md

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+# GPU Bitmap Best Fit Evaluation
+
+## Overview
+
+Add GPU-accelerated bitmap-based overlap testing to the best fit pair evaluation pipeline using ILGPU. Parts are rasterized to integer grids; overlap detection becomes cell comparison on the GPU. Runs alongside the existing geometry-based evaluator, selectable via flag.
+
+## Architecture
+
+New project `OpenNest.Gpu` (class library, `net8.0-windows`). References `OpenNest.Core` and `OpenNest.Engine`. NuGet: `ILGPU`, `ILGPU.Algorithms`.
+
+## Components
+
+### 1. `Polygon.ContainsPoint(Vector pt)` (Core)
+
+Ray-cast from point rightward past bounding box. Count edge intersections with polygon segments. Odd = inside, even = outside.
+
+### 2. `PartBitmap` (OpenNest.Gpu)
+
+- Rasterizes a `Drawing` to `int[]` grid
+- Pipeline: `ConvertProgram.ToGeometry()` -> `Helper.GetShapes()` -> `Shape.ToPolygonWithTolerance()` -> `Polygon.ContainsPoint()` per cell center
+- Dilates filled cells by `spacing / 2 / cellSize` pixels to bake in part spacing
+- Default cell size: 0.05"
+- Cached per drawing (rasterize once, reuse across all candidates)
+
+### 3. `IPairEvaluator` (Engine)
+
+```csharp
+interface IPairEvaluator
+{
+    List<BestFitResult> EvaluateAll(List<PairCandidate> candidates);
+}
+```
+
+- `PairEvaluator` — existing geometry path (CPU parallel)
+- `GpuPairEvaluator` — bitmap path (GPU batch)
+
+### 4. `GpuPairEvaluator` (OpenNest.Gpu)
+
+- Constructor takes `Drawing`, `cellSize`, `spacing`. Rasterizes `PartBitmap` once.
+- `EvaluateAll()` uploads bitmap + candidate params to GPU, one kernel per candidate
+- Kernel: for each cell, transform to part2 space (rotation + offset), check overlap, track bounding extent
+- Results: overlap count (0 = valid), bounding width/height from min/max occupied cells
+- `IDisposable` — owns ILGPU `Context` + `Accelerator`
+
+### 5. `BestFitFinder` modification (Engine)
+
+- Constructor accepts optional `IPairEvaluator`
+- Falls back to `PairEvaluator` if none provided
+- Candidate generation (strategies, rotation angles, slide) unchanged
+- Calls `IPairEvaluator.EvaluateAll(candidates)` instead of inline `Parallel.ForEach`
+
+### 6. Integration in `NestEngine`
+
+- `FillWithPairs()` creates finder with either evaluator based on `UseGpu` flag
+- UI layer toggles the flag
+
+## Data Flow
+
+```
+Drawing -> PartBitmap (rasterize once, dilate for spacing)
+               |
+Strategies -> PairCandidates[] (rotation angles x slide offsets)
+               |
+GpuPairEvaluator.EvaluateAll():
+  - Upload bitmap + candidate float4[] to GPU
+  - Kernel per candidate: overlap check + bounding box
+  - Download results
+               |
+BestFitFilter -> sort -> BestFitResults
+```
+
+## Unchanged
+
+- `RotationSlideStrategy` and candidate generation
+- `BestFitFilter`, `BestFitResult`, `TileEvaluator`
+- `NestEngine.FillWithPairs()` flow (just swaps evaluator)