v0.6.3

• Arrow matrix axes (#234): the C ABI Arrow export gains a table catalog and
  dense matrix axis maps. pio_to_arrow exposes matrix_bus and
  matrix_branch axis tables alongside the ybus, incidence, bprime, and
  bdoubleprime COO tables, which carry powerio.row_axis / powerio.col_axis
  schema metadata so a consumer maps dense matrix rows and incidence columns
  back to source bus and branch rows. Matrix rows are labeled with the
  matrix_bus axis, which stays correct when 3-winding transformer star-point
  lowering expands the bus set past the handle bus order.
• FDPF matrices (#234): bprime / bdoubleprime follow MATPOWER makeB
  semantics, with self-loop handling and asymmetric Matrix Market writes pinned
  by Rust, C ABI, and Python coverage.
• Summary JSON (#234): the C ABI exposes balanced (pio_summary_json) and
  distribution summary JSON, so a binding can render network summaries without
  materializing the full model payload.
• Matrix Arrow export (#234): the numeric matrix export path fills Julia-owned
  buffers directly for the copy-free fast path.

What's Changed

• [codex] Add Arrow catalog and matrix axes by @samtalki in #234

Full Changelog: v0.6.2...v0.6.3