docs: add NFP best-fit integration design spec

Spec for adding NFP-based placement as a competing strategy
in NestEngine.FindBestFill() for non-rectangular parts.

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

docs/superpowers/specs/2026-03-13-nfp-bestfit-design.md

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+# NFP Strategy in FindBestFill
+
+## Problem
+
+`NestEngine.FindBestFill()` currently runs three rectangle-based strategies (Linear, RectBestFit, Pairs) that treat parts as bounding boxes. For non-rectangular parts (L-shapes, circles, irregular profiles), this wastes significant plate area because the strategies can't interlock actual part geometry.
+
+The NFP infrastructure already exists (used by `AutoNest`) but is completely separate from the single-drawing fill path.
+
+## Solution
+
+Add `FillNfpBestFit` as a new competing strategy in `FindBestFill()`. It uses the existing NFP/BLF infrastructure to place many copies of a single drawing using actual part geometry instead of bounding boxes. It only runs when the part is non-rectangular (where it can actually improve on grid packing).
+
+## Design
+
+### New method: `FillNfpBestFit(NestItem item, Box workArea)`
+
+Located in `NestEngine.cs`, private method alongside `FillRectangleBestFit` and `FillWithPairs`.
+
+**Algorithm:**
+
+1. Compute `halfSpacing = Plate.PartSpacing / 2.0`
+2. Extract the offset perimeter polygon via `ExtractPerimeterPolygon(drawing, halfSpacing)` (already exists as a private static method in NestEngine)
+3. **Rectangularity gate:** compute `polygon.Area() / polygon.BoundingBox().Area()`. If ratio > 0.95, return empty list — grid strategies already handle rectangular parts optimally
+4. Compute candidate rotation angles:
+   - Start with hull edge angles via `ComputeHullEdgeAngles(polygon)` (already exists in NestEngine)
+   - Always include 0° and 90°
+   - Filter by `NestItem.RotationStart` / `NestItem.RotationEnd` window (keep angles where `RotationStart <= angle <= RotationEnd`; if both are 0, treat as unconstrained 0–360°)
+5. Build an `NfpCache`:
+   - For each candidate rotation, rotate the polygon and register it via `nfpCache.RegisterPolygon(drawing.Id, rotation, rotatedPolygon)`
+   - Call `nfpCache.PreComputeAll()` — since all entries share the same drawing ID, this computes NFPs between all rotation pairs of the single part shape
+6. For each candidate rotation, run `BottomLeftFill.Fill()`:
+   - Build a sequence of N copies of `(drawing.Id, rotation, drawing)` where N = `(int)(workArea.Area() / polygon.Area())` capped to a reasonable max
+   - Place via BLF which uses IFP minus NFP unions to find valid positions
+7. Score each rotation's result via `FillScore.Compute(parts, workArea)`
+8. Return the parts list from the highest-scoring rotation, converted via `BottomLeftFill.ToNestParts()`
+
+### Integration into FindBestFill
+
+Insert after the Pairs phase, before remainder improvement, in both overloads of `FindBestFill`:
+
+```csharp
+// NFP phase (non-rectangular parts only)
+var nfpResult = FillNfpBestFit(item, workArea);
+Debug.WriteLine($"[FindBestFill] NFP: {nfpResult?.Count ?? 0} parts");
+
+if (IsBetterFill(nfpResult, best, workArea))
+{
+    best = nfpResult;
+    ReportProgress(progress, NestPhase.Nfp, PlateNumber, best, workArea);
+}
+```
+
+The progress-reporting overload also adds `token.ThrowIfCancellationRequested()` before the NFP phase.
+
+### NestPhase enum
+
+Add `Nfp` after `Pairs`:
+
+```csharp
+public enum NestPhase
+{
+    Linear,
+    RectBestFit,
+    Pairs,
+    Nfp,
+    Remainder
+}
+```
+
+## Files Modified
+
+| File | Change |
+|------|--------|
+| `OpenNest.Engine/NestEngine.cs` | Add `FillNfpBestFit()` method; call from both `FindBestFill` overloads after Pairs phase |
+| `OpenNest.Engine/NestProgress.cs` | Add `Nfp` to `NestPhase` enum |
+
+## What Doesn't Change
+
+- `FillBestFit`, `FillLinear`, `FillWithPairs` — untouched
+- `AutoNest` — separate code path, untouched
+- `BottomLeftFill`, `NfpCache`, `NoFitPolygon`, `InnerFitPolygon` — reused as-is, no modifications
+- UI callers (`ActionFillArea`, `ActionClone`, `PlateView.FillWithProgress`) — no changes, they call `NestEngine.Fill()` which calls `FindBestFill()` internally
+- MCP tools (`NestingTools`) — no changes
+
+## Edge Cases
+
+- **Part with no valid perimeter polygon:** `ExtractPerimeterPolygon` returns null → return empty list
+- **All rotations filtered out by constraints:** no BLF runs → return empty list
+- **BLF places zero parts at a rotation:** skip that rotation, try others
+- **Very small work area where part doesn't fit:** IFP computation returns invalid polygon → BLF places nothing → return empty list