powerio-dist: distribution crate integration

[samtalki]

Design a new crate powerio-dist that focuses on distribution network test cases. OpenDSS and all other distribution system data formats supported by PowerModelsDistribution.jl and PowerSystems.jl form a good starting point.

[samtalki]

PR #82 ships the crate this issue asked for. What landed:

• powerio-dist: a multiconductor model in wire coordinates (string bus ids, ordered terminal names, explicit grounding, terminal maps, SI and radians internally) with three formats: OpenDSS .dss, PowerModelsDistribution ENGINEERING JSON, and the draft BMOPF task force JSON schema (frederikgeth/bmopf-report).
• The workspace fidelity contract, extended with explicit defaults: same format writes echo the source byte for byte; cross format writes report every loss; the dss reader materializes each OpenDSS class default with provenance.
• Oracle gates: OpenDSS re-solve agreement to 1e-8 pu where conversion fidelity is the only variable, PMD parse_file acceptance of emitted JSON, schema validation (positive and negative) for BMOPF.
• All bindings: CLI powerio convert, Python powerio.dist, C ABI pio_dist_* behind a dist feature.

What this issue still covers beyond the PR:

• the deferred dss classes (powerio-dist: typed support for deferred OpenDSS classes #84)
• PowerIO.jl bindings (PowerIO.jl: bind the distribution C ABI #85)
• other formats named here: the PowerModelsDistribution math model JSON (the ENGINEERING serialization is what PR feat: powerio-dist, a multiconductor distribution crate with dss/PMD/BMOPF converters #82 reads and writes), PowerSystems.jl formats, CIM/RAVENS (MG-RAVENS: RAVENS JSON export and ingest backend #27), and whatever the task force schema becomes after PES GM. The BMOPF schema is a draft; the converter follows the public repo and the PR body carries a feedback list for the task force.