embed: ps_island.c engine seam for PyPy (init and exception args) #667

Plan: .agents/plans/2026-10-25-embedding-release-plan.md (branch work/embedplan). Tracking: #651. Epic: #428.

Size: M.

Two APIs, three lines in src/pyscriptc/runtime/ps_island.c: Py_InitializeEx(0) (line 37) becomes PS_ENGINE_START(), which for -DPS_ENGINE_PYPY is rpython_startup_code(), pypy_setup_home(home, 1) and pypy_init_threads() (order and the need for pypy_init_threads for PyGILState_Ensure from native threads were not run in the POC; verify first); PyException_GetArgs (lines 276 and 297) becomes PyObject_GetAttrString(val, "args"). The driver emits the define from the descriptor. Keep the PyGILState and SaveThread discipline from #174.

Acceptance: ps_island.c compiles against CPython and PyPy headers; the CPython island tests are unchanged; exception message tests green.

Depends on: #666.