v0.4.0

• powerio-pkg: .pio.json reads now enforce the envelope compatibility rule:
  same major schema_version values load, while incompatible major versions
  fail before payload use. The mdBook schema page documents the rule.
• powerio-pkg: balanced package output now emits source maps for stable bus,
  load, shunt, branch, and generator fields; validation diagnostics attach the
  matching source reference where a map exists. Format folded fields use
  mapping kinds such as split, and defaulted fields are not marked as exact
  source fields.
• Converter tests now compare stable per element values across writable legacy
  formats, not only counts and totals. PSLF export now warns when transformer
  charging admittance is dropped.
• powerio-dist BMOPF: OpenDSS fixed P/Q generators now emit as BMOPF
  generator.* entries with pinned P/Q bounds instead of negative load.*
  entries. The old negative load warning is gone; generators without source
  costs keep the existing cost 0 warning.
• Python API: removed the one release powerio.Case and
  powerio.dist.DistCase compatibility aliases. Use powerio.Network /
  powerio.BalancedNetwork and powerio.dist.MulticonductorNetwork /
  powerio.dist.DistNetwork.
• No C ABI rename in this migration slice: PIO_ABI_VERSION stays 4 and
  PIO_DIST_ABI_VERSION stays 1.

What's Changed

• [data] rich data model parity by @samtalki in #143
• [pkg] add .pio.json compiler package envelope by @samtalki in #156
• [codex] Add generator cost policy and package validation diagnostics by @samtalki in #154
• Consolidate docs into mdBook guide by @samtalki in #160
• Stabilize compiler package spine by @samtalki in #161
• Implement multiconductor lowering by @samtalki in #163
• Add MCP package transport by @samtalki in #162
• Add normalized solver table package metadata by @samtalki in #164
• [codex] Accept PSS/E Start of section markers by @samtalki in #165
• Add package C ABI surface by @samtalki in #166
• [codex] fix BMOPF generator export by @samtalki in #167
• [codex] prep v0.4.0 release by @samtalki in #168

Full Changelog: v0.3.3...v0.4.0