Add JMH benchmark suite

The current CLI's --count flag with --metrics (cli/README.md) produces rough timings, but it's not a substitute for proper microbenchmarks. Add a JMH-based benchmark module to measure hot paths and catch regressions.

• New :benchmarks Gradle module wired with the JMH plugin (excluded from the published artifacts).
• Cover the evaluator hot path: bundle load, plan prepare, single eval, and steady-state eval throughput.
• Compare JSON adapters (opa-gson vs opa-jackson) on the same inputs.
• Measure builtin invocation overhead (opa-builtins) — at least one hash, one regex, and one HTTP-free string builtin.
• Provide a small set of representative policies + inputs as fixtures so runs are reproducible.
• Document how to run (./gradlew :benchmarks:jmh) and how to interpret results in the module README.

Start by reading cli/README.md and inspecting the existing Gradle module configuration and published-artifact setup. Define the new :benchmarks module around the requested evaluator, adapter, and builtin cases, using representative policy and input fixtures. Done means ./gradlew :benchmarks:jmh runs reproducibly and the module README explains execution and result interpretation.