Private model-root and signed load gate

The independently generated NativeActivity host exposes CoreCLR's base directory as NativeHost.PrivateDataRoot(). Its native bootstrap supplies ANativeActivity.internalDataPath to CoreCLR as APP_CONTEXT_BASE_DIRECTORY. A rebuilt, locally signed APK was installed and launched on both attached devices; each logged that the resulting directory exists. The marker does not log the path.

Model.Store.psm1 now revalidates the signed manifest, runtime ABI, contract version, expiry, payload size, SHA-256, and managed assembly name when reading the active pointer. LoadActive loads only the result of that revalidation. The fresh Windows test passed signed admission and load, and rejected a modified stored manifest before load. Test-ProductionClosure.ps1 passed.

The rebuilt APK is 40,967,555 bytes, under the 40 MiB limit, with SHA-256 F59740E9F54255FF719366EB29D7281F9E49821FFD01B988FF1E5FCDE15C63F1. This APK remains model-less: the store source and a model-signing public key are not yet embedded, and the Android startup does not invoke LoadActive. The Windows test uses a test assembly, not a stock Kokoro synthesizer. No speech or on-device model-loading claim follows from this result.