Resource nodes break when worked out and grow back

From the alpha playtest (items 1, 4, 5): a chopped-out tree must be destroyed and grow back from a sapling, trees, rocks and veins must visibly take every stroke, the character must face what it works, and mining must work only on rocks and ore veins, never on bare rock terrain.

Built on fix/resource-nodes: strokes are object hits, worked-out nodes break and come back (nodes.rs), trees through sapling and young stages, durable across restarts; the tool in hand comes from items-corpses (#134). Open: tuning of the comeback times after the next wave.