[3.6] 3-wire decomposition (ZoR/ZoG/ZoB)

[RFingAdam]

Acceptance criteria

• Implement Y-decomposition per atlc2 docs §"3-wire transmission lines"
• Test against quarter-wave directional coupler reference values

Status: ❌ Not implemented (theory page exists; code does not)

[RFingAdam]

Closed — 3-wire Y-decomposition shipped in v1.1.0:

• B1: atlc3.analytical.three_wire closed-form Y-matrix decomposition
• B2: Boundary-weighted floating-conductor BC for the bitmap solver
• B3: Laplace.solve_modes extension via Y-matrix factorisation
  Tests in tests/test_analytical/test_three_wire.py. Theory doc docs/theory/three_wire.md.