srfake.New wraps an httptest.Server on a random port. That's ideal for unit
tests, but it means you can't serve the mock registry on a port you choose
(e.g. a fixed :8081 alongside a fake Kafka cluster).

This adds a small, backward-compatible way to bring your own listener:

• NewRegistry(opts ...Option) *Registry — builds a Registry without starting
  any server.
• (*Registry) Handler() http.Handler — returns the registry's HTTP handler so
  callers can serve it themselves.

New is unchanged and now builds on NewRegistry internally. For a
NewRegistry-created registry, URL() returns "" and Close() is a no-op;
all in-memory methods (SeedSchema, RegisterSchema, etc.) behave as before.

reg := srfake.NewRegistry()
ln, _ := net.Listen("tcp", "127.0.0.1:8081")
go http.Serve(ln, reg.Handler())
// ... reg.SeedSchema(...) etc. still work

Motivation: this lets a tool serve an in-process schema registry on a
configurable, fixed port in the same process as a kfake cluster (so e.g.
kcl fake can stand up brokers + registry together).