feat(benchmark): build jobs from DXF manifests and run solves in parallel

Benchmark jobs could only come from .nest files. A JSON manifest now lists
DXF files with quantities (plus sheet sizes, spacing, edge spacing, quadrant
and per-part allowRotation), imported through CadImporter. DXF paths resolve
relative to the manifest; sheet sizes are required from the manifest or
--sheet-sizes and are read in the DXFs' own units. Folder scans pick up
*.nest and *.manifest.json, and invalid manifests fail loudly.

BenchmarkRunner now runs (job x engine) solves concurrently, capped by
--parallel N (CLI default 3; --parallel 1 is sequential). Results are written
by index so report order is unchanged. Concurrent solves compete for cores,
so Time(ms) is only clean at --parallel 1; the run prints a note when N > 1.

Also fixes --output for manifest jobs, which tried to read the manifest as a
.nest to copy metadata from.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

OpenNest.Tests/OpenNest.Tests.csproj

@@ -23,6 +23,7 @@
 
   <ItemGroup>
     <ProjectReference Include="..\OpenNest.Api\OpenNest.Api.csproj" />
+    <ProjectReference Include="..\OpenNest.Benchmark\OpenNest.Benchmark.csproj" />
     <ProjectReference Include="..\OpenNest.Data\OpenNest.Data.csproj" />
     <ProjectReference Include="..\OpenNest.Core\OpenNest.Core.csproj" />
     <ProjectReference Include="..\OpenNest.Engine\OpenNest.Engine.csproj" />