feat(dist): add pre-solver electrical readiness audit

[BurhanAbdullah]

Summary

Add a read-only electrical readiness audit for multiconductor distribution networks before numerical lowering or solving.

What it checks

• invalid/non-finite base frequency
• duplicate bus, linecode, and line identifiers
• buses without terminals
• unresolved line bus/linecode references
• invalid line lengths
• line terminal-count versus linecode conductor-count mismatches
• zero-conductor linecodes
• malformed or non-finite linecode impedance/admittance matrices
• parser-recorded defaulted fields remain visible as readiness warnings

The audit is intentionally fail-closed for structural/numerical blockers and never mutates or silently repairs the network.

Why

This provides a reusable boundary between parsing and numerical use. In particular, it makes the existing defaulted provenance observable before a model reaches a solver/lowering path, complementing the deferred-geometry fail-closed work in #480/#484.

Tests

Unit tests are included for a valid network, unresolved linecode, and malformed impedance matrix. I have not claimed an executed workspace test run because the GitHub-connected environment does not provide a local Cargo build/test environment.