Auto CK root validators on subnet registration.

[shamil-gadelshin]

Summary

• Add do_set_root_validators_for_subnet function that automatically establishes parent-child relationships between all root network validators and a subnet owner's hotkey, enabling the subnet owner to inherit stake from root
  validators and meet the StakeThreshold required to call set_weights
• Integrate the function into do_register_network so relationships are scheduled automatically at subnet creation time
• Each root validator becomes a parent of the subnet owner hotkey on the new subnet with full proportion (u64::MAX), using the existing do_schedule_children flow (with PendingChildKeys cooldown)

New tests

• test_root_children_enable_subnet_owner_set_weights — verifies the function directly: subnet owner cannot set weights before, then can after root validator children are activated
• test_register_network_schedules_root_validators — verifies end-to-end from register_network: pending children are automatically scheduled during subnet registration, activate after cooldown, and the subnet owner can then set weights
  via inherited stake

[evgeny-s]

The implementation looks good, but the fact that we add subnet hotkey to all the root validators as a child without them knowing this not good, I think.

I think we might consider more options.

• We do it on subnet registration, but for a short period of time (N days)
• We require validator hotkey owners to confirm it during this period, otherwise it will be reverted back.

Another option for to providing the emission flow to the subnet, maybe it can be dedicated from the protocol, instead of using validators stake.

TDB

[open-junius]

need a different error type.

[shamil-gadelshin]

Seems close enough. Also, I don't expect errors here at all because it's called only during subnet registration with concrete netuid. What do you suggest instead?

[open-junius]

need a different error type.

[shamil-gadelshin]

Do we have a better one? It seems appropriate. We don't have subnet owner hotkey only when it doesn't exist. What do you suggest?