zk: allow extending own proposal without posting a new `initBond` to reduce proposer capital requirements

In the normal case where a chain operator is the main proposer they need to post `initBond` with every proposal. This causes high capital requirements for the operator of a chain with optimistic ZK where the bonds are locked up for multiple days.

A proposer should not require a bond to extend one of their own proposals (i.e. be able to extend their own chain without an additional bond). The chain operator's proposer will always be extending their own chain. And there's no incentive to post a chain of invalid proposals because one challenge defeats the whole chain anyway.

What makes this difficult is that an invalid game which extends a valid one by the same proposer provides no incentive to attack it since it has no bond.

Consider: `1 -> 2'`
- `1` is valid and has a bond, `2'` is invalid but has no bond since it extends a chain by the same proposer.

An attacker can only attack 2' but by doing so doesn't earn a reward.

Start by tracing how proposal extensions and `initBond` are handled, then read the surrounding optimistic-ZK game and challenge logic. Resolve how a bondless self-extension can still be challenged and rewarded appropriately; done means the capital requirement is reduced without removing incentives to attack invalid proposals.

Written by the indexing model from the issue text.