Benchmark/governance overhead (#792)

* feat(benchmarks): add governance overhead benchmark (#720)

- Benchmark script measuring p50/p95/p99 latency across all governance layers
- Real implementations: PolicyEvaluator, TrustPolicyEvaluator, AuditLog,
  CredentialManager, RingEnforcer, DeltaEngine
- 1000+ iterations per measurement using time.perf_counter()
- E2E governed vs ungoverned comparison
- Two charts: latency by component and overhead breakdown
- Full-stack governance overhead: 0.066ms p50 / 0.416ms p99
- Audit write (Merkle) dominates at 73% of total overhead

Closes #720

Signed-off-by: Anubhav Lakra <anubav@ramboll.dk>

* test(benchmarks): add unit tests for governance overhead benchmark

- 19 tests across 4 classes: TestBenchSync, TestGovernedVsUngoverned,
  TestSaveJson, TestGenerateCharts
- All pass in under 2 seconds using n=10 iterations
- Covers p50<=p95<=p99 invariants, governed>ungoverned latency,
  JSON output structure, and chart file generation

Signed-off-by: AnubhavLakra10 <a4anubhavlakra@gmail.com>

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Signed-off-by: Anubhav Lakra <anubav@ramboll.dk>
Signed-off-by: AnubhavLakra10 <a4anubhavlakra@gmail.com>

Author: AnubhavLakra10