v0.6.0

• Breaking (#175): ElementRef.row is Option<usize>, the honest form of the
  0.5.1 wire semantics. None addresses by identity alone (refs built with
  by_source_uid); the private wire-presence shim (wire_row()) is gone, and
  row itself says whether the wire carried one. The .pio.json wire format
  is unchanged. The other break collected in #175, keyed-object addressing for
  multiconductor operating point updates, needs design and moves to the 1.0
  window (#196).
• C ABI: the package payload extraction inverses land as additive symbols (no
  ABI version change; probe the symbols like the other feature surfaces):
  pio_package_to_balanced_network and pio_package_to_multiconductor_network
  materialize an owned network handle from a parsed .pio.json package handle,
  the inverses of the pio_package_from_* constructors. A handle built from a
  payload retains no source text, so a same-format write is a fresh
  serialization rather than a byte-exact echo; the multiconductor payload's
  parse warnings ride along.
• C ABI: pio_to_json / pio_from_json are the function form of the balanced
  model JSON (byte identical to the powerio-json writer); the format token
  remains as a compatibility alias for file based workflows. This is the
  additive half of #194; retiring the token waits for 1.0.
• C ABI: pio_dist_to_json / pio_dist_from_json serialize a distribution
  handle to its model JSON and back in one call each: the same object a
  .pio.json document carries under model.multiconductor_network, without
  the surrounding document. Bindings materialize element tables with this
  instead of building a throwaway package; it is not a case format (the
  converter, CLI, and inference do not know it), so BMOPF JSON remains the
  distribution JSON exchanged with other tools.
• C ABI: pio_classify_str classifies in-memory JSON by the same top level
  markers the transmission parser's .json sniffing uses, and recognizes
  .pio.json envelopes: transmission:<format>, distribution:<format>,
  package, ambiguous, or unknown, size-then-fill. Bindings can route a
  bare .json before choosing a parser instead of matching error text.
• The JSON classifier reports a .pio.json envelope as its own outcome
  (routing::JsonClass), so every consumer handles it: the CLI, the Python
  readers, and the Python classify_json_text now name the package surface
  for an envelope instead of a generic cannot-infer error (or, for the Python
  string reader, a MATPOWER syntax error). Envelope detection requires
  model_kind to be balanced or multiconductor, so a case document
  carrying those key names with other values still classifies as a case, and
  classification parses the document once.
• Directed errors at the transmission boundary: a .dss path, a distribution
  from token (dss/pmd/bmopf), and a .pio.json envelope handed to the
  balanced parser now name the surface that reads them instead of a generic
  unknown-format message.

What's Changed

• Document what the .pio.json format is and how it relates to BMOPF by @samtalki in #177
• Add design chapters for geographic data and the study block by @samtalki in #179
• Package payload inverses, JSON classifier entry point, directed boundary errors by @samtalki in #187

Full Changelog: v0.5.1...v0.6.0