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/OpenNest.Engine.Opus55.csproj

@@ -0,0 +1,14 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <RootNamespace>OpenNest.Engine.Opus55</RootNamespace>
+    <AssemblyName>OpenNest.Engine.Opus55</AssemblyName>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Remove="tests/**/*.cs" />
+    <InternalsVisibleTo Include="OpenNest.Engine.Opus55.Tests" />
+    <ProjectReference Include="../OpenNest.Engine/OpenNest.Engine.csproj" />
+  </ItemGroup>
+</Project>