feat: replace JMH with lightweight timing-based benchmarks

[Copilot]

fix: #502

Removes OpenJDK JMH dependencies (jmh-core, jmh-generator-annprocess) and replaces with a simple timing utility for benchmark tests.

Changes

• BenchmarkUtil: New utility class providing warmup/measurement phases with System.nanoTime() timing and throughput calculation
• Benchmark tests: Converted 4 test classes (46 benchmarks total) from JMH annotations to JUnit tests with timing wrapper

Example

Before (JMH):

@Benchmark
public void benchmarkRBACModel() {
    Enforcer e = new Enforcer("examples/rbac_model.conf", "examples/rbac_policy.csv");
    e.enforce("alice", "data2", "read");
}

After (BenchmarkUtil):

@Test
public void benchmarkRBACModel() {
    BenchmarkUtil.runBenchmark("RBAC Model", () -> {
        Enforcer e = new Enforcer("examples/rbac_model.conf", "examples/rbac_policy.csv");
        e.enforce("alice", "data2", "read");
    });
}

Output format remains similar (iterations, duration, ops/sec) with default 3 warmup + 5 measurement iterations.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.08%. Comparing base (eb4476f) to head (bdf68c0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #503      +/-   ##
==========================================
+ Coverage   67.92%   68.08%   +0.16%     
==========================================
  Files          63       63              
  Lines        3083     3083              
  Branches      574      574              
==========================================
+ Hits         2094     2099       +5     
+ Misses        805      801       -4     
+ Partials      184      183       -1     

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