feat: add _Template and New-Engine.ps1 for starting new engines

Each engine-building run started from a hand-copied scaffold (Qwen's),
which carried that engine's name and stale notes. _Template holds the
generic scaffold with a __NAME__ placeholder, and New-Engine.ps1 stamps
out a named copy. The template's starter tests go through the
benchmark's NestValidator, so a model gets a real pass/fail target
instead of a plumbing-only check; they were verified to pass against
Opus55.

Directory.Build.props now also detects when it sits in an Engines/
folder inside an OpenNest checkout, so an engine stamped there with
-IncludeBuildFiles builds without passing OpenNestRoot.

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

README.md

@@ -32,6 +32,23 @@ dotnet test OpenNest.Engine.Opus55/tests/OpenNest.Engine.Opus55.Tests.csproj
 
 Each engine's README covers its algorithm and benchmark results.
 
+## Writing a new engine
+
+```powershell
+./New-Engine.ps1 -Name Nova
+```
+
+This copies `_Template/` to `OpenNest.Engine.Nova/`: an `INestingEngine` stub, a README
+spelling out what counts as an independent engine, and starter acceptance tests checked by
+the benchmark's own `NestValidator` (they fail until `Solve()` is implemented).
+
+To work inside an OpenNest checkout instead, stamp it into an `Engines/` folder there and
+bring the shared build files along; they detect that layout automatically:
+
+```powershell
+./New-Engine.ps1 -Name Nova -Destination <OpenNest>/Engines -IncludeBuildFiles
+```
+
 ## Layout
 
 Each engine lives in `OpenNest.Engine.<Name>/` with an optional `tests/` subproject.