refactor: clean threaded runtime globals

[V3RON]

Why this matters

This removes undocumented secondary-runtime globals and fallback paths so runtime detection has one supported source of truth. It makes the public API clearer, avoids app code depending on internal globals, and keeps docs/examples aligned with the breaking cleanup from issue #12.

What changed

• kept global.__THREADED_RUNTIME_ENV__ as the only supported runtime identity global and reduced it to runtimeName and kind
• removed _is_it_a_list_env, __COMPOSE_CHAT_LIST_ENV__, __rnrRuntimeFunctionCacheRuntimeName, and the global event-emitter fallback guard
• removed native global / globalThis self-assignment during secondary runtime setup
• hid __rnrRegisterRuntimeFunction and __rnrCallRuntimeFunction behind internal helpers in core
• updated the example app to use getCurrentRuntime() and isMainRuntime() instead of raw globals
• updated README and website installation docs, including a short supported-vs-internal globals section

Verification

• bunx tsc --noEmit in example/
• bun test in example/ currently fails in the existing Bun + React Native test setup on Flow syntax in react-native/index.js