Curate the ccall layer (Clang.jl seed over powerio.h)

[samtalki]

Seed the raw ccall layer from powerio-capi/include/powerio.h with Clang.jl (a gen/ driver), then curate. The scaffold hand-wrote pio_parse/pio_to_json/pio_convert/pio_write_matpower + the CaseHandle finalizer — about 7 of the ~15 header symbols; this generalizes it (adds pio_from_json, pio_n_gens, pio_reference_bus, pio_n_components, pio_is_radial, and the numeric extractors) and keeps it in sync with the header. Maintenance hygiene; not release-blocking.

[samtalki]

The ccall surface expanded substantially in #10 — the dense extractors (pio_bus_ids / pio_branches / pio_gens / pio_nodal_*), the Arrow C Data Interface import (pio_export_arrow, decoded through hand-written ArrowArray/ArrowSchema FFI structs since Arrow.jl is IPC-only), and the pio_abi_version / pio_version handshake — all hand-written ccalls.

This issue stays open for the original intent: seed that layer from powerio.h with Clang.jl so the binding tracks the header automatically (new symbols, signature changes) rather than by hand. The hand-written surface is the reference for what the seed must cover.