0086 — Shield-generator build hours

Status

Folded into FEATURES/planets/citadels.md.

Context

Sets shield-generator upgrades on a time-based build (level × 6 h, stored on Planet.active_events and settled lazily on read), mirroring the defense-building construction queue.

The durable rule — the level × 6 h build-time table and the active_events lazy-settle storage — lives in FEATURES/planets/citadels.md. The numbers are operator-tunable.