YS v9.6.0 private Windows runtime directory

The launcher uses runtime\python312\python.exe if it is already present.
On a clean PC with no usable Python 3, RUN_YS_v960.bat automatically retrieves the official Python 3.12.10 Windows embeddable runtime into this directory, verifies it, and keeps it private to YS.

No system-wide Python installation, PATH edit, or administrator setup is intended.
