Mobile: load the 3.15t builds through the plugin's PYTHONHOME staging, run on simulator/emulator, then switch the mobile default #328

Follow-up of #163 (builds work; release cpython-3.15.0t-mobile). Per #158 (3.15t is the only build), mobile moves to 3.15t too.
Steps: (1) the Gradle plugin's PYTHONHOME staging and iOS consumer wiring (#90) and the Android staging can acquire the 3.15t archives (python-3.15.0t-aarch64-linux-android.tar.gz, python-3.15.0t-iOS-XCframework.tar.gz from the release, pinned SHA-256) and stage them (lib/python3.15t, libpython3.15t); (2) run on the iOS simulator (ios-consumer fixture and sample DEMO 1/8) and an Android emulator, Py_GIL_DISABLED observed at run time; (3) switch the mobile default to 3.15t, with the GIL build no longer offered on mobile.
Completion: simulator and emulator runs green on 3.15t through the plugin path only; the default is 3.15t on Android and iOS; docs/SPEC updated.