Desktop: find PYTHONHOME beside a native-image executable, and stage it there #246

A GraalVM native image has no java.home, no Compose resources directory and no launcher to set PYTHONHOME, so a packaged native image finds no CPython prefix unless its environment names one (found in #242; needed for #191's first milestone). The sample's native demo works only because its Gradle run task sets PYTHONHOME.

Change (SPEC L-9):

• Run time. PackagedPythonHome.resolve adds a fourth source after the existing three. Inside a native image (org.graalvm.nativeimage.imagecode is runtime), it uses <directory of the executable>/python-multiplatform-home when that directory exists. On a JVM the executable is java, so the rule does not apply there. The rest is unchanged: the environment still wins, and the prefix goes through L-3's check, Py_SetPythonHome and the libpython lookup like the other packaged sources.
• Build. The Gradle plugin registers stageNativeImagePythonHome, a Sync. It copies the same pruned prefix as Compose packaging (stdlib and shared library; no headers, bin/, Tcl/Tk or __pycache__) into <pythonBindings.nativeImageDirectory>/python-multiplatform-home, and does nothing unless that directory is set.

Completion: PM/desktopTest/.../PackagedPythonHomeTest pins the new resolution step (red first); GP/PackagedPythonHomeTest pins the task's wiring and its no-op default; the existing tests are unchanged.