feat: move deposit/withdrawal/stake processing into ConsensusState

[matthias-wright]

This reworks how Summit handles deposits, withdrawals, staking, and committee transitions: the logic moves out of the finalizer actor and into ConsensusState, making the state transition self-contained, deterministic, and unit-testable without the finalizer/EL harness.
This also addresses #339 and #362, superseding #379 and #397.

Changes:

• Move execution request (deposit, withdrawal, etc) processing from the finalizer actor into the ConsensusState.
• Execution requests are no longer parsed on arrival. They are only buffered.
• Withdrawal queue format was simplified, withdrawals are no longer keyed by validator key.
• Balance is no longer deducted when the withdrawal request arrives. The withdrawal amount is deducted from the balance only after the withdrawal lands in a block.
• Partial withdrawal requests for active validators are clamped to avoid withdrawing the balance below MinimumStake.
• Removed MaximumStake param, validators can stake as much as they want.
• When MinimumStake is increased, validators with balance < MinimumStake are still removed from the committee (if the resulting committee size is still larger than a defined minimum), but the remaining balance is no longer withdrawn automatically. Removed validators may withdraw their balance at any time.
• Removed the has_pending_deposit and has_pending_withdrawal fields from the validator accounts.
• Added FullPayoutPending to distinguish a validator that has left the committee with its full balance committed to a pending payout (must not rejoin) from a plain Inactive validator (out of committee, keeps its balance, may rejoin via a top-up and may hold a pending partial withdrawal). Without the FullPayoutPending, an incoming deposit would re-activate a validator whose entire balance is about to be withdrawn.
• Reject unverified-block withdrawals before EL forkchoice adoption.
• Clamp terminal-block payouts against a raised minimum.