refactor(engines): move plugin engines into an Engines/ subfolder

Plugin engines (Opus55, Qwen, Terra) each add two projects at the repo
root, and more are coming; at a dozen they would outnumber the core
projects. They are also a different kind of thing: out-of-solution,
runtime-loaded plugins. Grouping them under Engines/ keeps the root
readable.

Engines/Directory.Build.props now holds the shared TFM, nullable and
implicit-usings settings and the OpenNest.Engine reference, so a new
engine's csproj is nearly empty. The tests/ compile exclusion lives in
Directory.Build.targets because a removal in .props runs before the SDK
adds its default Compile glob and has no effect.

Build-Engines.ps1 replaces the per-README manual build-and-copy steps
for deploying engines into the benchmark's runtime Engines/ folder.

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

Build-Engines.ps1

@@ -0,0 +1,40 @@
+<#
+.SYNOPSIS
+    Builds every plugin engine under Engines/ and deploys it to the benchmark.
+
+.DESCRIPTION
+    OpenNest.Benchmark loads plugin engines from an Engines/ folder next to its own
+    build output. This builds the benchmark plus each Engines/OpenNest.Engine.*/ project
+    (test subprojects are skipped) and copies each engine DLL into that folder.
+
+.EXAMPLE
+    ./Engines/Build-Engines.ps1
+    ./Engines/Build-Engines.ps1 -Engines Opus55,Terra -Configuration Debug
+#>
+param(
+    [string]$Configuration = 'Release',
+    # Engine names without the OpenNest.Engine. prefix; default is all of them.
+    [string[]]$Engines
+)
+
+$ErrorActionPreference = 'Stop'
+$repoRoot = Split-Path $PSScriptRoot -Parent
+
+dotnet build (Join-Path $repoRoot 'OpenNest.Benchmark/OpenNest.Benchmark.csproj') -c $Configuration
+if ($LASTEXITCODE -ne 0) { throw 'OpenNest.Benchmark build failed.' }
+
+$deployDir = Join-Path $repoRoot "OpenNest.Benchmark/bin/$Configuration/net8.0/Engines"
+New-Item -ItemType Directory -Force $deployDir | Out-Null
+
+$projects = Get-ChildItem $PSScriptRoot -Directory -Filter 'OpenNest.Engine.*' |
+    Where-Object { -not $Engines -or $Engines -contains $_.Name.Substring('OpenNest.Engine.'.Length) }
+
+foreach ($dir in $projects) {
+    $csproj = Join-Path $dir.FullName "$($dir.Name).csproj"
+    dotnet build $csproj -c $Configuration
+    if ($LASTEXITCODE -ne 0) { throw "$($dir.Name) build failed." }
+
+    $dll = Join-Path $dir.FullName "bin/$Configuration/net8.0/$($dir.Name).dll"
+    Copy-Item $dll $deployDir -Force
+    Write-Host "Deployed $($dir.Name) -> $deployDir"
+}