Bevy revival mechanic (docs-first stub)

Not a full port. Docs-first stub. Godot counterpart: godot-mechanic-revival.

When to use

Load this when an actor is downed, can be revived, or respawns.

ECS shape

• Components: a downed flag and a revive elapsed you define.
• Resources: run-wide tables live in a resource (bevy-data-resources), not an autoload.
• Systems: mutate those components on Update or the fixed timestep. Use .chain() when damage must land before death.
• Messages: one-shot facts (hit, pickup, quest advance) go through bevy-events-messaging. This file does not add a new event type name.

Godot idea to Bevy idiom