Add manual OAS generation benchmark - #138

Problem

grape-oas profiles memory use during OAS generation, but it has no repeatable way to measure generation time as an API's route count grows. This makes performance work harder to evaluate across OAS dialects.

Fix

Add a manually invoked benchmark:generation Rake task inspired by Grape's compile_many_routes.rb. It builds synthetic APIs with 100, 500, and 1,000 routes, warms up each exporter, verifies the generated path count, and reports median generation time for OAS 2.0, 3.0, and 3.1.

Route counts and iteration count can be overridden with environment variables. The benchmark uses Ruby's monotonic clock, so it adds no dependency. It is not connected to CI or release tasks.

Example

bundle exec rake benchmark:generation
ROUTE_COUNTS=50,250 ITERATIONS=10 bundle exec rake benchmark:generation

Output is tab-separated for easy comparison:

routes  dialect  median_ms
100     oas2     6.79
100     oas3     6.72
100     oas31    6.82

Schema before / after

No change to emitted output. This PR adds a standalone benchmark that calls the existing GrapeOAS.generate API; library generation code is unchanged.

Backward compatibility

• Public API surface: unchanged.
• Emitted OAS shape: unchanged.
• New runtime/dev dependencies: none.

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