Add PRV_Cron_Guard: idempotent init-hook guard that re-schedules
prv_weekly_probe and prv_daily_prune when missing (e.g. after
an rsync deploy that bypasses the activation hook).
- New class: includes/core/class-prv-cron-guard.php
- Hooks on init; two wp_next_scheduled() checks on hot path
- Delegates to PRV_Cron::schedule() and PRV_Prune_Cron::schedule()
so recurrence + offset are always canonical (no inline divergence)
- Idempotent: no-op when events already scheduled
- PRV_Plugin::init(): wires PRV_Cron_Guard::register_hooks()
- Tests: tests/unit/test-cron-guard.php (missing, idempotent, recurrence)
- Bump version 0.3.0 -> 0.3.1 (header + PRV_VERSION constant + bootstrap)
- CHANGELOG: [0.3.1] entry
a23644b1af