feat(mcp): add verifier latency observer

[brooksmcmillin]

Summary

• add an optional latency_observer callback to MCPVerifier
• measure end-to-end verification latency from method entry across every allow and denial return path
• isolate observer failures and mutations by passing a deep-copied result snapshot after trusted control-plane emission
• feed the same end-to-end duration into existing control-plane telemetry
• document a Prometheus histogram adapter

Test plan

• ruff check tenuo/mcp/server.py tests/adapters/test_mcp_server.py
• uv run --no-sync python -m pytest tests/adapters/test_mcp_server.py tests/adapters/test_control_plane.py -q (84 passed, 5 skipped)
• git diff --check

Closes #496

Task: https://nexus.brooksmcmillin.com/task/1688