Add PowerModelsDistribution.jl support for unbalanced distribution networks #14

Context

PowerModelsDiff currently supports single-phase balanced transmission networks via PowerModels.jl. PowerModelsDistribution.jl (PMD) is listed in Project.toml (v0.16.0) but is not used anywhere in the codebase.

This issue scopes the effort to extend PowerModelsDiff to unbalanced multi-phase distribution networks.

Key Barriers

• Data format differences: PMD uses its own ENGINEERING/MATHEMATICAL data models with per-phase vectors and matrix impedances, rather than PowerModels' scalar bus/branch format
• Per-phase matrices: Sensitivity matrices become block-structured (3×3 per element pair for three-phase). Incidence matrix A, admittance/susceptance data, and the Laplacian all need multi-phase generalizations
• Expanded KKT system: The OPF KKT system grows significantly with per-phase variables, constraints, and cross-phase coupling (mutual impedances)
• Topology: Distribution networks are typically radial, which may allow specialized (more efficient) sensitivity computations vs. the general meshed-network approach

Milestones

1. Data parsing: DistributionNetwork type(s) that wrap PMD data into the PowerModelsDiff type hierarchy, handling phase indexing, multi-phase admittance matrices, and transformer models
2. Unbalanced power flow sensitivities: Per-phase voltage and current sensitivities, either by extending DCPowerFlowState / ACPowerFlowState or creating distribution-specific variants
3. Distribution OPF sensitivities: KKT-based implicit differentiation for the multi-phase OPF formulation, reusing the caching patterns from DCSensitivityCache / ACSensitivityCache