docs(template): add BENCH-RULES.md for engine-building runs

Rules for the runs were being made up mid-run (git for rollback, access
to the real-part drawing archive), so each model started under a
different rule set. BENCH-RULES.md writes them down in one place, stamped
into every new engine, so all models work under the same rules:
workspace limits and no searching for other engines, git init plus
commit-per-working-state, the archive as read-only with nothing
customer-identifying kept in the (publishable) engine folder, tests may
only be added to, and what the final report must cover.

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

README.md

@@ -38,7 +38,8 @@ Each engine's README covers its algorithm and benchmark results.
 ```
 
 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
+spelling out what counts as an independent engine, `BENCH-RULES.md` (how a model's run
+works: workspace limits, git, the real-part archive, reporting), 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