Order Change Substrate

Every post-placement change to an order (a return, an exchange, a claim, an admin editing line items, a draft order being amended) is the same shape: someone proposes a set of changes, somebody reviews the financial consequence, and the changes are applied atomically or thrown away. This introduces one shared substrate, Spree::OrderChange plus Spree::OrderChangeAction, that all of them build on: a pending change set that can be previewed and then confirmed in a single transaction. The 6.0 returns, exchanges and claims flows adopt it without a breaking API change.

Plan: https://github.com/spree/spree/blob/main/docs/plans/6.1-order-change-substrate.md

Tracked in Linear as V-3616.