Test durable signer and observer recovery

Signer and observer crashes, retries, and retiring-vault transitions need durability and single-broadcast guarantees. This change adds recovery fault tests and fixes durable-write ordering.

Validation: Local race and recovery checks passed; integrated Bifrost race and regression CI jobs passed. This topic is also included in draft !5181 (closed). This MR will run its own CI against develop; review and merge coordination should avoid landing the same change twice.