Implement RequestStakeWithdrawal #210

Scope

Require the actor's signature, mark the bond as exiting, and record the start of the configured exit delay. Exiting actors cannot take on new work; the request does not release locked stake.

This remains v2 lifecycle work, at lower priority than the core commitment and challenge flow.

Design: v2 implementation notes.

Outcome

Allow a bonded operator or verifier to start stake withdrawal through RequestStakeWithdrawal.