Design record drafted: fixes to stateful products are migrations

A new design record is drafted and open for review. It covers how fixes are handled for stateful products. A product here is a set of contracts with its own repository and a manual that describes it, and it is stateful when those contracts hold state. The record's position is that a fix to a stateful product is a migration, and that a fix builds from the product's own repository and lands back in it.

Under the draft, a product's manual declares how each of its contracts can change: stateless, upgradeable, migratable or fixed. The check that validates a manual refuses a manual that leaves a fund-holding contract stateless.

The fix path follows that declaration. A stateless contract is redeployed. An upgradeable one gets an upgrade payload. A migratable one goes through a migration that takes two votes, and the product's operations pause for the migration and resume with it.

The record and its supporting research notes are committed on a branch, along with updated indexes for the design records. The pull request is waiting for review and has not been merged.