perf: eliminate inspect imports from protocol.py, repr.py, and syntax.py

- syntax.py: move Console, ConsoleOptions, JustifyMethod, RenderResult
  to TYPE_CHECKING (already has future annotations). Eliminates
  rich.console from syntax's import chain entirely.
- protocol.py: replace `from inspect import isclass` with
  `isinstance(x, type)` — identical behavior, no import needed.
- repr.py: defer `import inspect` into auto_rich_repr() where
  inspect.signature() is actually used.

None of these save time yet (inspect still comes in via dataclasses),
but they become free wins once dataclasses is removed from console.py.