Document graph-to-matrix quantization and reduction pipeline

[joanjcaceres]

Summary

Add a theory document for the graph/lumped-circuit layer that prepares the reduced matrices and branch data consumed by BBQ.

Motivation

Before implementing graph-to-matrix numerical code, SCCircuits needs a mathematical guide for the construction. The existing BBQ docs now define the backend boundary; this issue documents what happens before that boundary.

Documentation scope

• Branch orientation conventions and incidence matrices B_C, B_L, and B_J.
• External flux matrix S and the distinction between independent loop fluxes and per-branch offsets.
• Construction of capacitance, inverse-inductance, Josephson, and source terms such as C, L_inv, and I_ext from graph data.
• Classification of variables as frozen, free, periodic, and extended.
• Frozen-variable elimination by constraints / Schur complement.
• Free-variable treatment by conserved charge sector / Routhian.
• Output contract into BBQ: reduced capacitance_matrix, reduced inverse_inductance_matrix, junction records, and per-junction external phase offsets.

Acceptance criteria

• A new theory page explains the mathematical pipeline without adding runtime graph-layer code.
• The page is linked from the docs navigation.
• The page states what is normative for future implementation and what is intentionally deferred.
• pixi run -e sccircuits docs-build passes.

Notes

This issue should not include private Obsidian note contents or local paths. It can use the same notation as the BBQ theory page where possible.