Signet.

Defines signet, a kind of test network where blocks need a signature as well as mining, so testing is steadier.

Testnet lets people try things without real money, but it is unreliable, with long gaps and sudden bursts of blocks. On a signet, blocks must be signed by keys chosen when the network is set up, so they arrive more predictably. This suits long-running tests between several organisations, such as exchanges.