docs(engines): template on shared services and a determinism rule

New engines start from the shared test kit (the template's tests are a
one-line EngineContractTests subclass) and the host APIs, so they don't
re-derive geometry reading, work areas or validator tolerances.
BENCH-RULES.md now forbids clocks, unseeded randomness and environment
variables from influencing placement (budgets count work; wall time only
through the host's cancellation token) and lists the kit as read-only.
Build-Engines.ps1 deploys only OpenNest.Engine.* folders, and
New-Engine.ps1 -IncludeBuildFiles copies the kit.

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

_Template/BENCH-RULES.md

@@ -41,6 +41,12 @@ file covers how the run itself works.
 
 ## Tests and scoring
 
+- `Engine.Testing/` is the shared, read-only test kit supplied beside engine folders in
+  bench copies. Reference it for shapes, job construction, layout assertions and inherited
+  engine contract tests; do not copy or edit it during an engine optimization run.
+- Placement must be deterministic: no clocks, unseeded randomness or environment variables
+  may influence placement. Budgets count work. Wall time may stop work only through the
+  host's cancellation token. Diagnostics must not affect placement decisions.
 - Keep the starter tests in `tests/` and keep them passing. Add tests; do not weaken,
   skip or delete existing ones. If you believe an existing test is wrong, leave it and explain
   why in your report.