Remove state() from the public API #101

The maintainer decided (2026-10-04, relayed by the ecosystem lead) that state() goes: Kotlin's mutable_state_of does the same job. Remove it from the public API, with no alias and no compatibility shim (AGENTS.md §7: the rule wins).

Callers move to the notebook's way (remember_saveable inside App) or to mutable_state_of:

• the guide (guide-composables.html);
• SPEC;
• the stub generator and stubs (def state(initial));
• the tests (tests/test_app_root.py).

The app_root state stays an internal function (_new_state), never a public name.

Done when from pythonx.compose.runtime import state fails, the stubs have no state, app_root still works, and the suite passes in both environments.