Test job was wall-clocked by `internal/server/http` (~156s on the
self-hosted runner under -race). Two changes here cut that:
1. Matrix-shard the test job by package group: server-http, store,
and "rest" (everything else, computed via `go list | grep -v`).
Each shard runs on its own runner so the heavy package isn't
CPU-starved by siblings.
2. `auth.HashPassword` drops to cheap argon2id params (8 KiB / 1
iter / 1 lane) when `testing.Testing()` returns true. Production
params are unchanged. VerifyPassword reads params from the
encoded hash so cheap-params hashes verify identically — no test
call sites need to change.
steve