Retro compatibilty suite

[klaidliadon]

This PR adds a retro compatibility test suite that provides a server that uses in memory storage and various clients with different versions.

Current implementation only includes the latest version.

[klaidliadon]

Superseded by #147, which builds on the same idea but with key improvements:

• Actually tests an older release: includes a v0.23.3 pinned client (separate Go module, GOWORK=off) that exercises the legacy middleware chain against the current server — proving wire compatibility across the type renames
• Measures usage deltas instead of absolute values, so clients run sequentially without interference
• mock.NewServer Options pattern (RedisClient, Logger) allows sharing a real Redis between server and test harness without miniredis
• CI workflow (.github/workflows/retro.yml) runs both clients end-to-end with Redis service container
• Stacked on the v2 branch (refactor(quota-v2): type renames #144 → feat(quota-v2): server plumbing #145 → feat(quota-v2): cache middleware #146 → feat(quota-v2): retro compat #147), so it validates the actual v2 changes rather than master