start_client(name: X) and child_spec(name: X) were documented but ignored
:name. start_client/1 called Client.start_link/1 (single arg) which used
the implicit __MODULE__ default; child_spec/1 used :name only as the
supervisor :id while the start MFA still resolved to the single-arg form.
A second call collided with :already_started, and :id vs registered name
diverged uselessly.

API changes:
- KafkaEx.API.start_client/1 extracts :name from opts and passes it as
  the second arg to Client.start_link/2. Defaults to :no_name (unnamed)
  when :name is absent — eliminates the implicit-global-registration
  footgun. :name accepts atom, {:global, term}, or {:via, Module, term}.
- KafkaEx.API.child_spec/1 threads :name into the start MFA tuple so
  the spawned child registers under the requested name.
- KafkaEx.Client.start_link/2 @SPEC widened from atom to
  GenServer.name() | :no_name — the runtime already accepted any of
  these shapes via GenServer.start_link/3's name: option.

State cleanup:
- Drop the vestigial state.worker_name field. It was set once in init/1
  and read only by a Logger.debug in terminate/2. Honors the original
  reporter's "the name of the worker and genserver should never cross
  paths" by removing the redundant identity. State.static_init/2 →
  static_init/1.
- terminate/2 log now reports the registered name (via
  Process.info(self(), :registered_name)) for named clients, falling
  back to self() for unnamed.

Test coverage (test/integration/lifecycle/start_client_test.exs, 8 tests
+ doctest):
- default no-:name returns unnamed pid; Process.whereis(KafkaEx.Client)
  is nil
- name: nil equivalent to omitting :name
- atom name registers locally
- two clients with distinct atom names coexist
- name: {:global, term} registers globally
- name: {:via, Registry, {reg, key}} works
- child_spec/1 produces the expected :id + start MFA shape
- supervised children with distinct :name run independently; killing
  one triggers supervisor restart under the same registered name, and
  the restarted client serves API.metadata/1 cleanly

Refs #416.