Add an in-memory optimistic-concurrency contract fixture

[jonathanmcmichael]

Outcome

Provide a test-only in-memory ProjectRepository proving the current optimistic-version contract before a PostgreSQL driver or schema is chosen.

Truth source

Synthetic sequence: initial save with no expected version returns version 1; save with expected version 1 returns version 2; a second save expecting version 1 returns VersionConflict and does not change stored content.

Invariants

• the fake implements the public repository port rather than creating another API
• project UUID is the lookup identity
• conflicts never partially write
• no database URL, driver, or schema choice enters the fixture

Expected scope

Add a persistence truth fixture or focused test support under rustit-fixtures and exercise the asynchronous port deterministically.

Non-goals

PostgreSQL, PostGIS, migrations, multi-project transactions, offline storage, and production locking.

Acceptance and evidence

• the three-step version sequence matches the truth source
• load returns the last successful project and version
• stale save leaves version and project unchanged
• missing project behavior is explicit
• cargo xtask verify passes

Dependencies: #10 and the existing ProjectRepository contract.