feat(sync): add ListBackend and its contract suite

[noctuum]

ListBackend reads an account's list and the entry for one work, and writes where the account stands on a work, adding the work to the list where it is not on it. A write is benshi_core::list::Write: status, progress and rewatches. It carries no score, so the list keeps the account's own. Every call carries the backend's deadline.

• benshi_sync::Error has six variants: Timeout, Unreachable, Wait, Refused, NoWork and Unreadable. A message is a reason and names no backend.
• benshi_sync::memory::Memory keeps a list and the works off it, and fails in any of five ways on request.
• tests/contract holds the suite, in the shape of the PlayerWatcher suite. Memory passes it, and contract_rejects.rs has a backend for each clause that breaks that clause alone.

cargo test --workspace passes, 868 tests. The suite runs on a paused clock, so a deadline of 15 s costs no real time.