Docs: name the host entry point PythonAppView, not PythonContent #92

python-multiplatform #105 (landed in #167) restored the host API's original name; PythonContent no longer exists. The contract line is now PythonAppView(module = "pythonx.compose.runtime", attribute = "app_root"). Rename it in SPEC S5.4, the guide, the runtime docstrings and the test docstring. Docs and comments only; counts must not change. Also record that the root-swap entry point (#11) is the binder's: PythonAppView reads the app_root State inside the composition, and the PMP #26 diagnosis showed a redeclared root redraws.