v3.2.6-sr94.15: Private destination health monitoring

Adds opt-in owner-scoped destination monitoring with desktop/mobile controls, revision-safe APIs, asynchronous HEAD checks, strict public-only DNS/TLS policy, crash recovery, private status events and aggregate operator metrics. Existing links are unmodified and monitoring defaults off. Full isolated regression and rendered workflows pass; production/restore acceptance remains pending in docs/FEATURE-ROADMAP.md. See docs/DESTINATION-HEALTH.md for limits, authorization, monitoring and recovery restrictions.