Context

Part of #685 (Wave 3, P2). Reference-tier hardening.

Acceptance Criteria

• roadmap to consolidate Result/event_bus dual representation (header facade + split files + compat shim + parallel .cppm)
• make module build silently-disabled-on-AppleClang/non-Ninja visible as explicit skip-with-warning in CI

Derived from the 2026-05-30 multi-agent cross-system level-matching analysis (workflow wf_0a968699-e07). Reference systems: documentation=common_system, code-structure=monitoring_system, tests/examples=container_system.