feat(engine): Opus55 frontier-advance NFP nesting engine

Rename the OpenNest.Engine.Sonnet5 scaffold to OpenNest.Engine.Opus55 and
implement an independent whole-job INestingEngine (no built-in engine,
registry, or best-fit internals are called or copied).

- PartCatalog: snapshot perimeter -> polygon per allowed orientation, with
  adaptive chord tolerance and MBR-aligned rotations for Automatic parts.
- NoFitCache: spacing footprints and cached Clipper2 Minkowski NFPs (convex
  fast path; concave sweep plus both containment terms).
- FrontierPacker: per-(type, orientation) free regions (inner-fit rectangle
  minus NFPs), updated incrementally; gap-fill-largest, else least front
  advance per area^beta.
- Engine: look-ahead stock choice by estimated whole-job net area, six
  deterministic variants, tail re-plan of the last 1-3 sheets.
- Tests judged by OpenNest.Benchmark's NestValidator, including an NFP
  containment regression guard.

P260805-10.nest (219 parts), all 9 stock sizes: 219/219 valid, 27 sheets,
91.7% utilization, ~7 s.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.Engine.Opus55/SheetEconomics.cs

@@ -0,0 +1,41 @@
+using OpenNest.Engine.Jobs;
+
+namespace OpenNest.Engine.Opus55;
+
+/// <summary>
+/// The objective the engine optimizes: sheet area consumed, less the salvage credit for the
+/// single largest full-width or full-length edge offcut the job's options allow. Packing toward
+/// one edge (see <see cref="PackAxis"/>) is what makes that offcut large.
+/// </summary>
+internal static class SheetEconomics
+{
+    public static double SheetArea(NestPlateStock stock) => stock.Size.Width * stock.Size.Length;
+
+    public static double NetArea(NestJobOptions options, SheetFill fill)
+    {
+        var area = SheetArea(fill.Stock);
+        var minimum = options.MinimumSalvageDimension;
+        if (options.SalvageRate <= 0 || minimum <= 0 || fill.Parts.Count == 0)
+            return area;
+
+        var work = FrontierPacker.WorkArea(fill.Stock);
+        var gap = fill.Stock.PartSpacing;
+        var left = fill.Parts.Min(p => p.Left);
+        var right = fill.Parts.Max(p => p.Right);
+        var bottom = fill.Parts.Min(p => p.Bottom);
+        var top = fill.Parts.Max(p => p.Top);
+        var offcuts = new[]
+        {
+            // Box.Length is the X extent, Box.Width the Y extent.
+            (work.Length, bottom - work.Bottom - gap),
+            (work.Length, work.Top - top - gap),
+            (left - work.Left - gap, work.Width),
+            (work.Right - right - gap, work.Width),
+        };
+        var salvage = 0.0;
+        foreach (var (a, b) in offcuts)
+            if (a >= minimum && b >= minimum)
+                salvage = System.Math.Max(salvage, a * b);
+        return area - options.SalvageRate * salvage;
+    }
+}