docs: add NFP best-fit strategy design spec

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/superpowers/specs/2026-03-20-nfp-bestfit-strategy-design.md

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+# NFP-Based Best-Fit Strategy
+
+## Problem
+
+The current best-fit pair generation uses `RotationSlideStrategy`, which samples Part2 positions by sliding it toward Part1 from 4 directions at discrete step sizes. This is brute-force: more precision requires more samples, it can miss optimal interlocking positions between steps, and it generates hundreds of candidates per rotation angle.
+
+## Solution
+
+Replace the slide-based sampling with NFP (No-Fit Polygon) computation. The NFP of two polygons gives the exact mathematical boundary of all valid positions where Part2 can touch Part1 without overlapping. Every point on that boundary is a guaranteed-valid candidate offset.
+
+## Approach
+
+Implement `NfpSlideStrategy : IBestFitStrategy` that plugs into the existing `BestFitFinder` pipeline. No changes to `PairEvaluator`, `BestFitFilter`, `BestFitResult`, tiling, or caching.
+
+## Design
+
+### New class: `NfpSlideStrategy`
+
+**Location:** `OpenNest.Engine/BestFit/NfpSlideStrategy.cs`
+
+**Implements:** `IBestFitStrategy`
+
+**Constructor parameters:**
+- `double part2Rotation` — rotation angle for Part2 (same as `RotationSlideStrategy`)
+- `int type` — strategy type id (same as `RotationSlideStrategy`)
+- `string description` — human-readable description
+
+**`GenerateCandidates(Drawing drawing, double spacing, double stepSize)`:**
+
+1. Extract perimeter polygon from the drawing inflated by `spacing / 2` using `PolygonHelper.ExtractPerimeterPolygon` (shared helper, extracted from `AutoNester`)
+2. Create a rotated copy of the polygon at `part2Rotation` using `PolygonHelper.RotatePolygon` (also extracted)
+3. Compute `NoFitPolygon.Compute(stationaryPoly, orbitingPoly)` — single call
+4. If the NFP is null or has fewer than 3 vertices, return empty list
+5. Walk the NFP boundary:
+   - Each vertex becomes a `PairCandidate` with that vertex as `Part2Offset`
+   - For edges longer than `stepSize`, add intermediate sample points at `stepSize` intervals along the edge (catches optimal positions on long straight NFP edges)
+6. Return the candidates list
+
+### Shared helper: `PolygonHelper`
+
+**Location:** `OpenNest.Engine/BestFit/PolygonHelper.cs`
+
+**Static methods extracted from `AutoNester`:**
+- `ExtractPerimeterPolygon(Drawing drawing, double halfSpacing)` — extracts and inflates the perimeter polygon
+- `RotatePolygon(Polygon polygon, double angle)` — creates a rotated copy normalized to origin
+
+After extraction, `AutoNester` delegates to these methods to avoid duplication.
+
+### Changes to `BestFitFinder.BuildStrategies`
+
+Replace `RotationSlideStrategy` instances with `NfpSlideStrategy` instances. Same rotation angles from `GetRotationAngles(drawing)`, different strategy class. No `ISlideComputer` dependency needed.
+
+```csharp
+private List<IBestFitStrategy> BuildStrategies(Drawing drawing)
+{
+    var angles = GetRotationAngles(drawing);
+    var strategies = new List<IBestFitStrategy>();
+    var type = 1;
+
+    foreach (var angle in angles)
+    {
+        var desc = $"{Angle.ToDegrees(angle):F1} deg NFP";
+        strategies.Add(new NfpSlideStrategy(angle, type++, desc));
+    }
+
+    return strategies;
+}
+```
+
+### No changes required
+
+- `PairEvaluator` — still evaluates candidates (overlap check becomes redundant but harmless and fast)
+- `BestFitFilter` — still filters results by aspect ratio, plate fit, etc.
+- `BestFitResult` — unchanged
+- `BestFitCache` — unchanged
+- Tiling pipeline — unchanged
+- `PairsFillStrategy` — unchanged
+
+## Edge Sampling
+
+NFP vertices alone may miss optimal positions along long straight edges. For each edge of the NFP polygon where `edgeLength > stepSize`, interpolate additional points at `stepSize` intervals. This reuses the existing `stepSize` parameter meaningfully — it controls resolution along NFP edges rather than grid spacing.
+
+## Files Changed
+
+| File | Change |
+|------|--------|
+| `OpenNest.Engine/BestFit/NfpSlideStrategy.cs` | New — `IBestFitStrategy` implementation |
+| `OpenNest.Engine/BestFit/PolygonHelper.cs` | New — shared polygon extraction/rotation |
+| `OpenNest.Engine/Nfp/AutoNester.cs` | Delegate to `PolygonHelper` methods |
+| `OpenNest.Engine/BestFit/BestFitFinder.cs` | Swap `RotationSlideStrategy` for `NfpSlideStrategy` in `BuildStrategies` |
+
+## What This Does NOT Change
+
+- The `RotationSlideStrategy` class stays in the codebase (not deleted) in case GPU slide computation is still wanted
+- The `ISlideComputer` / GPU pipeline remains available
+- `BestFitFinder` constructor still accepts `ISlideComputer` but it won't be passed to NFP strategies (they don't need it)