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