powerio docs

Reference material that does not fit in the top level README or a crate doc comment.

• format-fidelity.md: numeric conventions every reader and writer follows, how they're validated against four independent tools, and the per-format limits reported in Conversion::warnings.
• matrices.md: the matrix family powerio-matrix builds and the sign, tap, per unit, and DC conventions across them.
• dcopf-bundle.md: the Matrix Market + manifest schema the dcopf subcommand writes for a downstream solver.
• languages.md: canonical Rust, Python, Julia, and C ABI names.
• python.md: Python install extras and API examples.
• Julia bindings: https://github.com/eigenergy/PowerIO.jl.

Rendered API docs (rustdoc) for all crates: https://eigenergy.github.io/powerio/.

Architecture

Network is the canonical model: format neutral, with loads, shunts, branches, and generators as first-class records. Every reader produces a Network and every writer consumes one, so a format is one reader/writer at the hub rather than a pairwise converter, and adding one touches a single module. IndexedNetwork is the dense [0, n) analysis view derived from a Network; the matrix builders work from it. The parser, the hub, the source retaining writer, and the converters live in the powerio crate (light dependencies); the matrices and graph views live in powerio-matrix, which re-exports powerio so one import pulls in both layers.